JavaScript Flashcards
What is Scope?
Refers to the availability of variables in our code
or
Scope dictates where JS engine should look for things
What is the scope of Var?
function scoped
What is the scope of let and const ?
block scoped
How are var declarations hoisted?
var declarations when hoisted they are initialized with undefined.
How are let and cont declarations hoisted?
when hoisted they are not initialized with undefined.
So, this would trigger a reference error if accessing before declaration.
What is Hoisting?
Hoisting means moving variable declarations to the top of their scope.
What are your use cases for variable declarations?
Try to use const declarations
If the value is expected to change, use let declaration
var declarations can be avoided as they introduce unnecessary confusion
Can you redeclare variables with the let keyword
No, this is not possible.
Error: Syntax Error: Identifier ‘name’ has already been declared
Does a function have access to global scope?
Yes, a function will always have access to its own function scope as well as the global scope.
A function can access variables defined with in the function as well as outside the function
What is Lexical Scoping?
means that a variable defined outside a function can be accessible inside another function defined after the variable declaration. But the opposite is not true; the variables defined inside a function will not be accessible outside that function.
‘this’ refers to it’s current surrounding scope and no further.
What is Scope Chain?
When we have nested functions, the JS engine for variable lookup starts with the inner most function where we are trying to access the variable and moves outward until it reaches the global scope
What is important to remember about Scope and nested functions?
Nested function have access to variables declared in their own scope as well as variables declared in the outer scope
What are Closures?
A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment).
In JavaScript, closures are created every time a function is created, at function creation time.
Further info:
This would let the function definition have an associated persistent memory which could hold on to live data between executions
What is Memoization
An optimization technique used primarily to speed up computer programs by storing the results of expensive function calls and returning the cached results when the same inputs occur again
What is Function Currying?
Currying is a process in functional programming in which we transform a function with multiple arguments into a sequence of nesting functions that take one argument at a time
function fn(a,b,c) transformed into fn(a)(b)(c)
What is ‘this’?
The JavaScript this keyword refers to the object it belongs to. You can set it with bind() among others.
‘this’ keyword is the execution context for a function call
It allows you to introduce reusability by allowing this value to be dynamic based on how a function is invoked
What is default binding
If call, apply, bind are not set, JS will default to the global scope and set ‘this’ keyword to the window object.
What are the five ways to invoke a function?
Implicit binding, Explicit binding, ‘new’ binding, default binding and Lexical Binding
Using implicit Binding, how do we know what ‘this’ is referencing?
When the function in invoked with the dot notation, the object to the left if that dot is what the this keyword is referencing.
Ex. person.sayMyName( )
‘this’ keyword is referencing the ‘person’ object
What is the first num called?
function myFunc(num){
return num
}
myFunc(2)
Parameter
What is 2 called?
function myFunc(num){
return num
}
myFunc(2)
Argument
What is the difference between call and bind methods
instead of invoking the function, Bind returns a new function that you can invoke whenever you wish
What do you know about arrow functions?
Introduced in ES6
Concise Function syntax
Provides Lexical binding
Can’t be called with new keyword
Arrow functions do not bind their own “this” , instead, they inherit the one from the parent scope, which is called “lexical scoping”
What are Higher Order Functions?
This is a function that can take in another function
Ex. The .map() function is a higher order function that will run a callback on every item in the array and will push each result into a new array.
What are first class functions?
function that was built with the intention of being passed around to other functions. It does one specific thing, does not have side effects, and is not intended to be called directly, but rather, to be used by ‘other functions.’
What is Functional programming?
is the process of building software by composing pure functions, avoiding shared state, mutable data, and side-effects. Functional programming is declarative rather than imperative, and application state flows through pure functions.
What are Callbacks?
is a function passed into another function as an argument to be executed later.
What does the new keyword do?
- creates a new empty object { }
- sets the value of ‘this’ to be the new empty object
- calls the constructor method
In constructor, new keyword moves the this keyword from global object to inside the object
https://www.youtube.com/watch?v=HboT8g_QSGc
T or F does every Object have a Prototype property?
True
What is a promise
A promise is an object that may produce a single value some time in the future : either a resolved value, or a reason that it’s not resolved
What are the differences between == and === ?
a===b means that a and b are equal in both value and type.
a==b means that they are equal in value only.
Why use SASS?
SASS gives you access to variables and functions in CSS. Also it has a compiler that helps with debugging and you can nest classes.
What is the difference between null and undefined.
Undefined means a variable has been initialized but not been assigned a value; //var initialized let a; //this log will give undefined console.log(a)
null is an assignment
//var initialized and assigned to null let a = null; //log will give null console.log(a)
What is 403 error
The 403 status code, or a Forbidden error, means that the user made a valid request but the server is refusing to serve the request, due to a lack of permission to access the requested resource.
What is the virtual DOM?
A virtual DOM is a lightweight JavaScript representation of the DOM used in declarative web frameworks such as React, Vue.js, and Elm. Updating the virtual DOM is comparatively faster than updating the actual DOM, since nothing has to be rendered onto the screen.
What is deconstructing
Add Answer here
What is a class
Add Answer here
What is UseEffect
React Hook
Fires a function with the component first mounts
Why do we need react Keys
Add Answer here
What is the difference between apply and call binding?
Apply method accepts arguments in an array
Call method accepts arguments in strings
What are unit tests?
Testing code or a component in complete isolation
What are Integration Tests
Test how components work together and if they are working together properly.
End to End Testing
Simulates what the user is going to do.
Ex. From login to using all the features of the app.
What is Jest?
the testing library used by React Testing Library
A promise is always in one of three states. What are the three states?
Pending: which is initial state, neither fulfilled nor rejected
Fulfilled: meaning that the operation completed successfully
Rejected: meaning that the operation failed.
Why use Promises?
Promises help us deal with asynch code in a far more simpler way compared to callbacks
How do you change that status of a promise?
Through Resolve and Reject
resolve() - changes status from pending to fulfilled
reject() - changes status from pending to rejected
Can you directly mutate the state of a promise?
No.
What two methods/functions does the Promise Object provide access to?
.then() - called with promise.then()
.catch() - called with promise.catch()
Can Promises be chained?
Yes
What is Promise.all()
Allows us to query multiple APIs and preform some actions but only after all the APIS have finished loading
If one promise rejects, promise.all() will reject with an error message.
What does Promise.allSettled() do?
Promise.allSettled() method waits for all input promises to complete regardless of whether or not one of them is rejected
What does Promise.race() do?
Promise.race() method returns a promise that fulfills or rejects as soon as one of the input promises fulfills or rejects, with the value or reason from that promise.
What are Classes?
Classes are like a blue print. Ex. Class could be an email for several users.
Every user has a different value for an email and email is the class.
What is a constructor function?
Creates new objects based upon the Class
