W3 Flashcards
create an object named “obj” with data members “property_1: value_1” and “property_2:value_2” using object literal notation
var obj = { property_1: value_1,
property_2: value_2
};Add a function named “setAge” that sets the age of the object using the “newAge” parameter
var obj = { property_1: value_1,
property_2: value_2,
// ...,
"property n": value_n };
setAge: function(newAge){this.age = newAge}
Use the built in “Object.create()” function to create a new instance of an “architect” object named “architect1”
var architect1 = Object.create(architect);
call the “setName” function on an architect object named “arch2”. Set the name to “Mary”
arch2.setName(“Mary”);
create an “architect” object using a function constructor with the properties name, age, and occupation. inName and inAge are passed as arguments
function architect(inName, inAge){
this.name = inName;
this.age = inAge;
this.occupation = "architect";
}Use the architect function prototype to create a new method “setName” that takes the parameter “newName”
architect.prototype.setName = function(newName){this.name = newName};
Why won’t this code output the architect’s name property?
architect.prototype.outputNameDelay = function(){
setTimeout(function(){
console.log(this.name);
},1000);
}
// ...
architect2.outputNameDelay(); // outputs undefined
Because the “this” keyword is within the function scope of the “setTimeout” function, which isn’t a method of the architect object
To resolve this, we can create a function variable named “that” and initialize it to “this”. We can also use function arrow syntax.
Differences between initializing with var, let, and const
var : Scope is its current execution content (function scope if declared within a function, or global outside any function)
let: block scope
const: block scope, unmodifiable value
create a constructor within this class that sets the name and age of the object
class architect{
name; age; occupation = "architect"; // default value of "architect" for occupation
// CONSTRUCTOR HERE
setName(newName){this.name = newName}
setAge(newAge){this.age = newAge}
getName(){return this.name;}
getAge(){return this.age;}
}
constructor(setName = “”, setAge = 0){ // handle missing parameters
this.name = setName;
this.age = setAge;
}
Properties within classes are public by default. How do we declare a property as private?
Use the prefix ‘#’ before the identifier
How to catch an exception on a block of code
Use a try catch block. Example:
const PI = 3.14159;
console.log(“trying to change PI!”);
try{
PI = 99;
}catch(ex){
console.log("uh oh, an error occurred!");
}
console.log(“Alas, it cannot be done, PI remains: “ + PI); // This code runs successfully
Output an error message within a catch block
try{
PI = 99;
}catch(ex){
// Code here
}
console.log(ex.message);
How to execute code in a try catch block regardless of if the code runs successfully
add a “finally” block
ex.
try{
PI = 99;
}catch(ex){
console.log("uh oh, an error occurred: " + ex.message);
// outputs: uh oh, an error occurred: Assignment to constant variable.
}finally{
console.log("always execute code in this block");
}
How to create a custom exception with an error message
use the throw keyword
ex.
if(y == 0){
throw new Error("Division by Zero!");
}
How to guarantee the order of execution in a program (have functions run in the intended order)
Use the “promise” keyword within a function, then call the function followed by the “then” keyword
ex.
// output "A" after a random time between 0 & 3 seconds
function outputA(){
var randomTime = Math.floor(Math.random() * 3000) + 1;
return new Promise(function(resolve, reject){ // place our code inside a "Promise" function
setTimeout(function(){
console.log("A");
resolve(); // call "resolve" because we have completed the function successfully
},randomTime);
});
}
// call the outputA function and when it is “resolved”, output a confirmation to the console
outputA().then(function(){
console.log(“outputA resolved!”);
});
How to return data after a Promise
Pass the data to the resolve object, and output the data message
ex.
function outputA(){
var randomTime = Math.floor(Math.random() * 3000) + 1;
return new Promise(function(resolve, reject){ // place our code inside a "Promise" function
setTimeout(function(){
console.log("A");
resolve("outputA resolved!"); // call "resolve" because we have completed the function successfully
},randomTime);
});
}
// call the outputA function and when it is “resolved”, output a confirmation to the console
outputA().then(function(data){
console.log(data);
});
What keyword can we use in a promise if the function is not guaranteed to execute?
We can use “reject” instead of “resolve” and output the reason within a catch block
ex.
function outputA(){
var randomTime = Math.floor(Math.random() * 3000) + 1;
return new Promise(function(resolve, reject){ // place our code inside a "Promise" function
setTimeout(function(){
console.log("-");
reject("outputA rejected!"); // call "reject" because the function encountered an error
},randomTime);
});
}
// call the outputA function and when it is “resolved” or “rejected, output a confirmation to the console
outputA()
.then(function(data){
console.log(data);
})
.catch(function(reason){
console.log(reason);
});
How can we chain together Promise objects to execute one after the other?
using the “then” keyword
ex.
outputA()
.then(outputB)
.then(outputC)
syntax for arrow functions:
parameter => logic
When are arrow functions most useful when trying to simplify the syntax?
When the function takes no parameters.
ex.
var outputMessage = function() {
console.log("Hello Function Expression");
};
// is the same as:
var outputMessageArrow = () => console.log(“Hello Arrow Function”);
Explain these features of arrow functions:
Implicit return statement
Lexical “this”
Implicit return statement:
returns the value of the statement, without having to use the “return” keyword
ex. var adderArrow = (num1, num2) => num1 + num2;
Lexical “this”: Uses the “this” value of the parent scope.
ex.
architect.prototype.outputNameDelay = function(){
setTimeout(() => { console.log(this.name); }, 1000);
};
