JS Built-in Objects Flashcards
What is the global object in JavaScript?
JavaScript provides a global object which has a set of properties, functions and objects that are accessed globally, without a namespace.
List the global properties in JavaScript.
Infinity, NaN, undefined
List the global functions in JavaScript.
decodeURI(), decodeURIComponent(), encodeURI(), encodeURIComponent(), eval(), isFinite(), isNaN(), parseFloat(), parseInt()
List the global objects in JavaScript.
Array, Boolean, Date, Function, JSON, Math, Number, Object, RegExp, String, Symbol, and errors.
What does Infinity represent in JavaScript?
Infinity in JavaScript is a value that represents infinity.
How do you get negative infinity in JavaScript?
To get negative infinity, use the – operator: -Infinity.
What values are equivalent to positive and negative infinity?
Number.POSITIVE_INFINITY and Number.NEGATIVE_INFINITY
What happens when you add any number to Infinity, or multiply Infinity for any number?
It still gives Infinity.
What does NaN stand for?
Not a Number
What operations return NaN?
Operations such as zero divided by zero, invalid parseInt() operations, or other operations.
How do you check if a value evaluates to NaN?
You must use the isNaN() global function.
How do you check if a variable is undefined?
It’s common to use the typeof operator to determine if a variable is undefined.
What is the purpose of the decodeURI() function?
Performs the opposite operation of encodeURI()
What is the purpose of the decodeURIComponent() function?
Performs the opposite operation of encodeURIComponent()
What is the purpose of the encodeURI() function?
This function is used to encode a complete URL.
What characters does encodeURI() not encode?
It does encode all characters to their HTML entities except the ones that have a special meaning in a URI structure, including all characters and digits, plus those special characters: ~!@#$&*()=:/,;?+-_..
What is the purpose of the encodeURIComponent() function?
Instead of being used to encode an entire URI, it encodes a portion of a URI.
What characters does encodeURIComponent() not encode?
It does encode all characters to their HTML entities except the ones that have a special meaning in a URI structure, including all characters and digits, plus those special characters: _.!~*’()
What does the eval() function do?
This is a special function that takes a string that contains JavaScript code, and evaluates / runs it.
Why is the eval() function rarely used?
It can be dangerous.
What does the isFinite() function return?
Returns true if the value passed as parameter is finite.
What does the isNaN() function return?
Returns true if the value passed as parameter evaluates to NaN.
What is the purpose of the parseFloat() function?
Like parseInt(), parseFloat() is used to convert a string value into a number, but retains the decimal part.
What is the purpose of the parseInt() function?
This function is used to convert a string value into a number.
What is the second parameter of the parseInt() function?
The radix, always 10 for decimal numbers, or the conversion might try to guess the radix and give unexpected results.
What does parseInt() return if the string does not start with a number?
NaN (Not a Number)
What are the three ways to create an object in JavaScript?
Object literal syntax (const person = {}), Object global function (const person = Object()), Object constructor (const person = new Object())
What is the result of typeof {}?
object
What are static methods in the Object object?
Methods called directly on Object, not on an object instance.
What are instance methods in the Object object?
Methods called on an object instance.
What are the two properties of the Object object?
length (always 1) and prototype
What does Object.assign() do?
Copies all enumerable own properties from one or more objects to a target object (shallow copy).
What does Object.create() do?
Creates a new object with the specified prototype.
What does Object.defineProperties() do?
Creates or modifies multiple object properties at once.
What does Object.defineProperty() do?
Creates or modifies a single object property.
What does Object.entries() do?
Returns an array of an object’s enumerable property [key, value] pairs.
What does Object.freeze() do?
Freezes an object, preventing new properties from being added and existing properties from being removed or changed.
What does Object.getOwnPropertyDescriptor() do?
Returns a property descriptor for a named property on an object.
What does Object.getOwnPropertyDescriptors() do?
Returns all own property descriptors of an object.
What does Object.getOwnPropertyNames() do?
Returns an array of all own property names of an object, including non-enumerable properties.
What does Object.getOwnPropertySymbols() do?
Returns an array of all own Symbol property keys of an object.
What does Object.getPrototypeOf() do?
Returns the prototype (internal [[Prototype]]) of the specified object.
What does Object.is() do?
Determines whether two values are the same value.
What does Object.isExtensible() do?
Determines if an object is extensible (whether it can have new properties added to it).
What does Object.isFrozen() do?
Determines if an object is frozen.
What does Object.isSealed() do?
Determines if an object is sealed.
What does Object.keys() do?
Returns an array of a given object’s own enumerable property names.
What does Object.preventExtensions() do?
Prevents new properties from ever being added to an object.
What does Object.seal() do?
Prevents new properties from being added to an object and marks all existing properties as non-configurable.
What does Object.setPrototypeOf() do?
Sets the prototype (i.e., the internal [[Prototype]] property) of a specified object to a new prototype or null.
What does Object.values() do?
Returns an array of a given object’s own enumerable property values.
What does hasOwnProperty() do?
Returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it).
What does isPrototypeOf() do?
Returns a boolean indicating whether the object specified as the prototype of another object.
What does propertyIsEnumerable() do?
Returns a boolean indicating whether the specified property is enumerable and is the object’s own property.
What does toLocaleString() do?
Returns a locale-sensitive string representation of the object.
What does toString() do?
Returns a string representation of the object.
What does valueOf() do?
Returns the primitive value of the specified object.
What is a property descriptor object?
An object that defines a property’s behavior and attributes.
What are the properties of a property descriptor object?
value, writable, configurable, enumerable, get, set
What does the writable property of a descriptor do?
Determines if the property’s value can be changed.
What does the configurable property of a descriptor do?
Determines if the property can be removed or its attributes changed.
What does the enumerable property of a descriptor do?
Determines if the property appears in enumerations of the object’s properties.
What is the difference between Object.freeze() and Object.seal()?
Object.freeze() makes properties non-writable and non-configurable, while Object.seal() only prevents adding or removing properties.
What is the difference between Object.preventExtensions() and Object.seal()?
Object.preventExtensions() prevents adding new properties, while Object.seal() also prevents deleting existing properties.
What is the difference between Object.keys() and Object.getOwnPropertyNames()?
Object.keys() returns only enumerable property names, while Object.getOwnPropertyNames() returns all property names, including non-enumerable.
What is a shallow copy?
A copy where primitive values are cloned, but object references are copied (not the objects themselves).
How to check if an object is extensible?
Using Object.isExtensible()
How to check if an object is frozen?
Using Object.isFrozen()
How to check if an object is sealed?
Using Object.isSealed()
How do you create a number value using literal syntax?
const age = 36
How do you create a number value using the Number global function?
const age = Number(36)
What is the result of using the ‘new’ keyword with the Number function (e.g., new Number(36))?
A Number object is created.
How do you get the primitive number value from a Number object?
Using the valueOf() method.
What is Number.EPSILON?
The smallest interval between two representable numbers.
What is Number.MAX_SAFE_INTEGER?
The maximum safe integer value in JavaScript.
What is Number.MAX_VALUE?
The maximum positive representable value in JavaScript.
What is Number.MIN_SAFE_INTEGER?
The minimum safe integer value in JavaScript.
What is Number.MIN_VALUE?
The minimum positive representable value in JavaScript.
What is Number.NaN?
A special value representing ‘Not-a-Number’.
What is Number.NEGATIVE_INFINITY?
A special value representing negative infinity.
What is Number.POSITIVE_INFINITY?
A special value representing positive infinity.
What does Number.isNaN(value) do?
Returns true if the value is NaN, false otherwise.
What does Number.isFinite(value) do?
Returns true if the value is a finite number, false otherwise.
What does Number.isInteger(value) do?
Returns true if the value is an integer, false otherwise.
What does Number.isSafeInteger(value) do?
Returns true if the value is a safe integer, false otherwise.
What does Number.parseFloat(value) do?
Parses a string and returns a floating-point number.
What does Number.parseInt(value) do?
Parses a string and returns an integer.
What is a safe integer in JavaScript?
An integer that can be represented exactly as an IEEE-754 double-precision number.
What does .toExponential() do (instance method)?
Returns a string representing the number in exponential notation.
What does .toFixed() do (instance method)?
Returns a string representing the number in fixed-point notation.
What does .toLocaleString() do (instance method)?
Returns a string with a language-sensitive representation of the number.
What does .toPrecision() do (instance method)?
Returns a string representing the number to a specified precision.
What does .toString() do (instance method)?
Returns a string representation of the number in a specified radix.
What does .valueOf() do (instance method)?
Returns the primitive number value of a Number object.
What value does Number.NaN return for 0 / 0?
true
What is the default locale for toLocaleString() method?
US English
What is the default radix for Number.parseInt()?
10
What is the result of typeof new Number(36)?
object
What is the result of typeof Number(36)?
number
What is the static method of the String object used to create a string from Unicode characters?
String.fromCodePoint()
What does String.fromCodePoint(70, 108, 97, 118, 105, 111) return?
‘Flavio’
What does charAt(i) do?
Returns the character at index i.
What does charCodeAt(i) do?
Returns the Unicode 16-bit integer representing the character at index i.
What does codePointAt(i) do?
Returns the Unicode code point value at index i, handling characters outside the BMP.
What does concat(str) do?
Concatenates the current string with str.
What does endsWith(str) do?
Checks if a string ends with the value of str.
What does includes(str) do?
Checks if a string includes the value of str.
What does indexOf(str) do?
Returns the index of the first occurrence of str, or -1 if not found.
What does lastIndexOf(str) do?
Returns the index of the last occurrence of str, or -1 if not found.
What does localeCompare() do?
Compares a string to another according to locale, returning a number indicating their order.
What does match(regex) do?
Matches a string against a regular expression.
What does normalize() do?
Returns the string normalized according to a Unicode normalization form.
What does padEnd() do?
Pads the string with another string until it reaches a specified length, at the end.
What does padStart() do?
Pads the string with another string until it reaches a specified length, at the beginning.
What does repeat() do?
Repeats the string a specified number of times.
What does replace(str1, str2) do?
Replaces the first occurrence of str1 with str2, or all occurrences if using a global regex.
What does search(str) do?
Returns the index of the first match of str, or -1 if not found.
What does slice(begin, end) do?
Returns a new string from the portion between begin and end.
What does split(separator) do?
Splits a string into an array of substrings using the separator.
What does startsWith(str) do?
Checks if a string starts with the value of str.
What does substring(begin, end) do?
Returns a portion of a string between begin and end, handling negative indices differently than slice().
What does toLocaleLowerCase() do?
Returns a new string in lowercase according to locale.
What does toLocaleUpperCase() do?
Returns a new string in uppercase according to locale.
What does toLowerCase() do?
Returns a new string in lowercase.
What does toString() do?
Returns the string representation of the String object.
What does toUpperCase() do?
Returns a new string in uppercase.
What does trim() do?
Returns a new string with whitespace removed from both ends.
What does trimEnd() do?
Returns a new string with whitespace removed from the end.
What does trimStart() do?
Returns a new string with whitespace removed from the beginning.
What is the difference between charCodeAt() and codePointAt()?
charCodeAt() returns a 16-bit Unicode unit, while codePointAt() returns the full code point, handling characters outside the BMP.
How do you replace all occurrences of a string using replace()?
Use a regular expression with the global flag (/g).
What happens if charAt() is called with an index that does not exist?
An empty string is returned.
What does String.fromCodePoint(0x46, 0154, parseInt(141, 8), 118, 105, 111) return?
‘Flavio’
What is the difference between substring() and slice() when dealing with negative indexes?
substring() treats negative indexes as 0, while slice() counts from the end.
What is the default normalization form for normalize()?
NFC
What is the alias of trimEnd()?
trimRight()
What is the alias of trimStart()?
trimLeft()
What is Math.E?
The base of the natural logarithm (approximately 2.71828).
What is Math.LN10?
The natural logarithm of 10.
What is Math.LN2?
The natural logarithm of 2.
What is Math.LOG10E?
The base 10 logarithm of e.
What is Math.LOG2E?
The base 2 logarithm of e.
What is Math.PI?
The mathematical constant π (approximately 3.14159).
What is Math.SQRT1_2?
The reciprocal of the square root of 2.
What is Math.SQRT2?
The square root of 2.
Are Math object methods static or instance methods?
Static methods.
What does Math.abs(x) do?
Returns the absolute value of x.
What does Math.acos(x) do?
Returns the arccosine of x (in radians).
What does Math.asin(x) do?
Returns the arcsine of x (in radians).
What does Math.atan(x) do?
Returns the arctangent of x (in radians).
What does Math.atan2(y, x) do?
Returns the arctangent of the quotient of its arguments (y/x).
What does Math.ceil(x) do?
Rounds x up to the nearest integer.
What does Math.cos(x) do?
Returns the cosine of x (in radians).
What does Math.exp(x) do?
Returns e raised to the power of x.
What does Math.floor(x) do?
Rounds x down to the nearest integer.
What does Math.log(x) do?
Returns the natural logarithm (base e) of x.
What does Math.max(x1, x2, …) do?
Returns the largest of zero or more numbers.
What does Math.min(x1, x2, …) do?
Returns the smallest of zero or more numbers.
What does Math.pow(x, y) do?
Returns x raised to the power of y.
What does Math.random() do?
Returns a pseudorandom number between 0 and 1.
What does Math.round(x) do?
Returns the value of x rounded to the nearest integer.
What does Math.sin(x) do?
Returns the sine of x (in radians).
What does Math.sqrt(x) do?
Returns the square root of x.
What does Math.tan(x) do?
Returns the tangent of x (in radians).
What is the range of values accepted by Math.acos() and Math.asin()?
-1 to 1.
What is the return unit of trigonometric functions like Math.cos(), Math.sin(), Math.tan(), Math.acos(), Math.asin(), and Math.atan()?
Radians.
What kind of number does Math.random() return?
A pseudorandom floating-point number between 0 (inclusive) and 1 (exclusive).
What is JSON?
A file format used to store and interchange data in key-value pairs.
Is JSON human-readable?
Yes.
How are keys represented in JSON?
Wrapped in double quotes.
What separates a key and its value in JSON?
A colon (:).
What separates key-value pairs in JSON?
A comma (,).
Does spacing matter in a JSON file?
No.
In what year was JSON born?
2002
What is the MIME type for JSON files?
application/json.
What are the basic data types supported by JSON?
Number, String, Boolean, Array, Object, null.
How are strings represented in JSON?
Wrapped in double quotes.
How are arrays represented in JSON?
Wrapped in square brackets ([]).
How are objects represented in JSON?
Wrapped in curly brackets ({}).
What does the null keyword represent in JSON?
An empty value.
What JavaScript object provides methods for encoding and decoding JSON?
The JSON object.
What method is used to parse a JSON string into a JavaScript object?
JSON.parse().
What method is used to convert a JavaScript object into a JSON string?
JSON.stringify().
What is the purpose of the optional second argument in JSON.parse()?
It’s a reviver function used to perform custom operations during parsing.
How can you represent nested objects in JSON?
By including objects within other objects or arrays.
How are boolean values represented in JSON?
true or false (lowercase).
How are numbers represented in JSON?
Without quotes.
What standard defines JSON?
ECMA-404.
What file extension is commonly used for JSON files?
.json
How do you initialize a Date object representing the current time?
new Date()
What unit of time does the Date object use internally?
Milliseconds since January 1, 1970 (UTC).
How do you create a Date object from a UNIX timestamp?
new Date(timestamp * 1000)
What does new Date(0) represent?
January 1, 1970 00:00:00 UTC.
How does the Date object handle strings passed to its constructor?
It uses the parse method.
What does Date.parse(‘2018-07-22’) return?
A timestamp (milliseconds).
How do you create a Date object using year, month, and day parameters?
new Date(year, month, day)
What is the minimum number of parameters for a Date constructor with year, month, etc.?
3 (year, month, day)
How are months numbered in the Date object?
Starting from 0 (January is 0).
How do you specify a timezone when initializing a Date object?
By adding ‘+HOURS’ or ‘(Timezone Name)’ to the date string.
What happens if you specify a wrong timezone name?
JavaScript defaults to UTC without error.
What does Date.now() return?
The current timestamp in milliseconds.
What does Date.UTC(2018, 6, 22) return?
A timestamp in milliseconds representing July 22, 2018 UTC.
What does date.toString() return?
A string representation of the date and time.
What does date.toUTCString() return?
A string representation of the date and time in UTC.
What does date.toISOString() return?
A string representation of the date and time in ISO 8601 format.
What does date.getTime() return?
The timestamp in milliseconds.
What does date.getDate() return?
The day of the month.
What does date.getDay() return?
The day of the week (0-6, Sunday is 0).
What does date.getFullYear() return?
The full year.
What does date.getMonth() return?
The month (0-11).
What does date.getHours() return?
The hours.
What does date.getMinutes() return?
The minutes.
What does date.getSeconds() return?
The seconds.
What does date.getMilliseconds() return?
The milliseconds.
What does date.getTimezoneOffset() return?
The timezone difference in minutes.
What is the difference between get* and getUTC* methods?
get* methods return values based on the local timezone, getUTC* methods return UTC values.
What methods are used to edit a Date object?
setDate(), setFullYear(), setMonth(), setHours(), setMinutes(), setSeconds(), setMilliseconds(), setTime()
What is the deprecated method to set the year?
setYear()
How do you get the timestamp in seconds from Date.now()?
Math.floor(Date.now() / 1000)
What is the unary operator that can be used to get the timestamp?
+
What happens if you overflow a month with the days count?
The date will go to the next month.
What object is used to format dates according to the locale?
Intl.DateTimeFormat()
How do you compare two dates?
By comparing their getTime() values.
How do you check if two dates are equal?
By comparing their getTime() values.
How do you determine if a Date object represents today?
By comparing getDate(), getMonth(), and getFullYear() with today’s date.
What are the four ways to create a new Date object?
No parameters (now), number (milliseconds), string (date), parameters (parts of date).
What does date.toLocaleTimeString() return?
A string with a language-sensitive representation of the time portion of this date.
What does date.toLocaleString() return?
A string with a language-sensitive representation of this date.
What is the Intl object in JavaScript?
A built-in object that provides language-sensitive string comparison, number formatting, date and time formatting, and more.
What properties does the Intl object expose?
Intl.Collator, Intl.DateTimeFormat, Intl.NumberFormat, Intl.PluralRules, Intl.RelativeTimeFormat.
What method does the Intl object provide?
Intl.getCanonicalLocales().
What does Intl.getCanonicalLocales() do?
Checks if a locale is valid and returns the correctly formatted locale.
What happens if Intl.getCanonicalLocales() is passed an invalid locale?
It throws a RangeError.
How can you handle an invalid locale error?
Using a try/catch block.
Which String prototype method interfaces with the Intl API?
String.prototype.localeCompare().
Which Number prototype method interfaces with the Intl API?
Number.prototype.toLocaleString().
Which Date prototype methods interface with the Intl API?
Date.prototype.toLocaleString(), Date.prototype.toLocaleDateString(), Date.prototype.toLocaleTimeString().
What does Intl.Collator provide?
Language-sensitive string comparison.
How do you initialize a Collator object?
new Intl.Collator(locale).
What method is used to compare strings with a Collator object?
compare().
What does the compare() method return?
A positive, negative, or zero value indicating the order of the strings.
What does Intl.DateTimeFormat provide?
Language-sensitive date and time formatting.
How do you initialize a DateTimeFormat object?
new Intl.DateTimeFormat(locale).
What method is used to format a date with a DateTimeFormat object?
format().
What does the formatToParts() method return?
An array with all the date parts.
What does Intl.NumberFormat provide?
Language-sensitive number formatting, including currency.
How do you initialize a NumberFormat object for currency?
new Intl.NumberFormat(locale, { style: ‘currency’, currency: ‘XXX’ }).
What property is used to set the minimum number of fraction digits in NumberFormat?
minimumFractionDigits.
What does Intl.PluralRules provide?
Language-sensitive plural formatting and plural language rules.
How do you initialize a PluralRules object?
new Intl.PluralRules(locale, { type: ‘ordinal’ }).
What method is used to select the plural form with a PluralRules object?
select().
What are some practical usages of Intl.PluralRules?
Giving a suffix to ordered numbers (e.g., 1st, 2nd, 3rd).
What is the use of Intl.RelativeTimeFormat?
“provides language-sensitive relative time formatting”
What is a Set in JavaScript?
A collection of unique values.
How do you initialize a Set?
new Set()
How do you add an item to a Set?
set.add(value)
Does a Set allow duplicate values?
No.
How do you check if a Set contains an item?
set.has(value)
How do you delete an item from a Set?
set.delete(value)
How do you find the number of items in a Set?
set.size
How do you delete all items from a Set?
set.clear()
How do you iterate over the items in a Set?
Using set.keys(), set.values(), set.entries(), set.forEach(), or a for…of loop.
How do you initialize a Set with values?
new Set([value1, value2, …])
How do you convert a Set to an array?
[…set.keys()] or […set.values()]
What is a Map in JavaScript?
A collection of key-value pairs.
How do you initialize a Map?
new Map()
How do you add an item to a Map?
map.set(key, value)
How do you get an item from a Map?
map.get(key)
Does a Map allow duplicate keys?
No.
Does a Map allow duplicate values?
Yes.
How do you delete an item from a Map?
map.delete(key)
How do you delete all items from a Map?
map.clear()
How do you check if a Map contains an item by key?
map.has(key)
How do you find the number of items in a Map?
map.size
How do you initialize a Map with values?
new Map([[key1, value1], [key2, value2], …])
Can objects be used as keys in a Map?
Yes.
What does map.get(nonExistentKey) return?
undefined
How do you iterate over the keys of a Map?
Using map.keys() and a for…of loop.
How do you iterate over the values of a Map?
Using map.values() and a for…of loop.
How do you iterate over the key-value pairs of a Map?
Using map.entries() or map directly in a for…of loop.
How do you convert the keys of a Map to an array?
[…map.keys()]
How do you convert the values of a Map to an array?
[…map.values()]
