JavaScript
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What is JavaScript?
JavaScript is a single-threaded, multi-paradigmed (OOP and FP), interpreted, programming language
What is the difference between let, const, and var?
let and const are blocked scoped. var is function scoped no matter how deeply it may be nested inside that function.
What is the difference between a first-class and higher-order function?
A first-class function is a function that can be used just like any other variable. For example, a first-class function can be assigned to a variable, passed as an argument to another function, and even even be returned by a function. A higher-order function is a subset of a first-class function; it can only be passed to and returned by other functions
What is JS’s most popular execution environment?
The browser
What are the 7 primitive types and what are their associated bits?
• number (64 bit double) • bigInt (arbitrary bit integer) • boolean • string (16 bit unicode) • null • undefined
What is the result of the following code: NaN === NaN
The result is false. This is because NaN is not equal to any other value, including itself. Use Number.isNaN(x) instead
What is the behavior of === for primitives and reference types?
Primitives: • Test to see if 2 values are the same Reference Types: • Test to see if 2 reference variables point to the same object in the heap
Are strings iterable?
Yes
What are the 2 ways to access a character of a string at a given index?
1. str.charAt(index); 2. str[index];
Code an example of a template literal
let name = "Gianmarco"; Console.log(`Hello, ${name}!`)
Which 5 values evaluate to false and which values evaluate to true?
Evaulate to false: * null * undefined * NaN * 0 * “” Evaluates to true: * Everything else
What is the global object for all JS code inside a browser?
Window
What is the default value for uninitialized variables?
undefined
Code an example of a destructuring assignment
let [x, y] = [1, 2]; // variable names do not have to match console.log(x, y); // 1 2 let {x, y} = {x: 0, y:0} // variable names must match, else undefined console.log(x, y) // 0 0
Code an example of the 2 different ways to access an object’s property
let obj = {x:0, y:0}; console.log(obj["x"]); // 0 console.log(obj.x); // 0
What is the behavior of optional chaining? a?.b;
If a is not null or undefined, access b. Else, return undefined
All objects are instances of which class?
Object
What do the following expressions return? 1 && 2; 0 && 1; 1 && 0;
1 && 1 returns 2. This is because if both values are true, the right operand is returned 0 && 1 returns 0. This is because if the first value is false, the expressions short circuits and returns it 1 && 0 returns 0. This is because 1 is true so the operator returns the right operand
Code a function using a default parameter value
function f(x = 0) {...}
What is the behavior of the nullish coalescing operator? a ?? b;
If a is null or undefined, return b. Else, return a
What does the following code return? let result = "x" in {x: 0}; console.log(result);
function
What is the behavior of the following code? let obj = {x:0, y:0}; delete obj.x; console.log(obj);
Deletes the x property of obj {y:0}
What is the behavior of the following code? let arr = [1,2,3]; delete arr[0]; console.log(arr.length); console.log(typeof arr[0]);
Deletes the first element of arr causing it to become a sparsed array. It’s length will remain at 3 but its first element will be undefined
Code 2 examples using the in operator. 1 for an array, and the other for an object
let arr = ["a","b"]; console.log(0 in arr); // true let obj = {x:0, y:0}; console.log("x" in obj); //true