Javascript basics

James Drummond
Flashcards by James Drummond, updated more than 1 year ago
James Drummond
Created by James Drummond over 5 years ago


A-Level Javascript Flashcards on Javascript basics, created by James Drummond on 12/16/2014.

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Question Answer
What does 'new' do in Javascript? 1. A new object is created, inheriting from parent.prototype. 2. The Constructor function is called with the specified arguments and this bound to the newly created object 3. The object returned by the constructor function becomes the result of the whole new expression. If the constructor function doesn't explicitly return an object, the object created in step 1 is used instead.
What is hoisting in Javascript? All of the declarations are hoisted to the top of the scope
When you declare a variable and assign a value and it's hoisted, is the value hoisted with the declaration? No. Only the declaration is hoisted.
What are the five 'falsey' values in Javascript? undefined, null, NaN, 0, '', false
What are some features of Ecmascript 6? - Maps, sets and associated functions - Templates - Modules, classes - Make inheritance simpler
How do you test if something is an array? - IE9+ : isArray() - If variable.constructor === Array - If variable instanceof Array
What are the four types of function invocation in Javascript? - method invocation: obj.method(); - function invocation: method(); - constructor invocation: new Obj(); - call(), apply()
What is a closure An 'immediate' or 'self invoking' function whose private variables retain their value after the function has returned.
What are the datatypes in Javascript? String Number Boolean Object Array
What is the quickest way of finding an element in an array? .indexOf() - Uses strict equality - Not supported by all browsers
How do you compare objects for equality in Javascript? - You can JSON.stringify the objects and compare the strings _.isEqual() - angular.equals(o1, o2)
What is a polyfill? Code or plugin that enables you to use un-supported functions natively
What is the purpose of Object.create()? - Use it to instantiate a subclass. - Takes prototype as argument - Usually assign the result to the prototype of the subclass
What are some built in methods of Object? - toString() - isPrototypeOf() - hasOwnMethod() - hasOwnProperty() - constructor
Why would you use hasOwnProperty()? If you were iterating the properties of an object: for(var prop in obj){ if (obj.hasOwnProperty(prop){ // property not on prototypal chain } } - could make sure this object had the specific property
What are some built in methods of Array? - pop() - push() - reverse() - toString() - join() - slice() - splice() - reverse() - sort() indexOf()
What is typeof null? object
What is typeof NaN? number
What is typeof null? Object
What does 'use strict' do? Enforces strict syntax: will not allow: - undeclared variables - redeclaration of variables
What are some bad things about Javascript? - implied globals - No inbuilt modularity - forgetting 'new' keyword
How do you determine if an object is a prototype of another object? newObject.isPrototypeOf(oldObject)
How do you add two arrays together in Javascript? var newArray = array1.concat(array2)
How would you create a subclass in javascript with Object.create()? function Rectangle() Rectangle.prototype = Object.create(Shape.prototype) Rectangle.constructor = Shape.constructor
How do you create a subclass using new? function Child(arg1, arg1){ Parent.apply(this, args); }; Child.prototype = new Parent()
What is the constructor property in Javascript? It's a reference to the Constructor function. Practically it is not used for very much
How do you insert a character into a string in Javascript? Remember that Javascript strings are immutable var myStr = 'hi there'; var newString = myStr.subStr(0, i) + 'u' + myStr.subStr(i + 1);
How do you remove an item from the beginning of an array? var fruits = ["Apple", "Banana"]; fruits.shift();
How do you add an item to the beginning of an array? fruits.unshift()
How do you find the index of an item in an array? var fruits = ["Apple", "Banana", "mango"]; var index = fruits.indexOf('mango');
How do you remove an item from an array? var pos = 1; var removedItem = fruits.splice(pos, 1); // Remove one item from position one
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