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Diferences between variables undefined, variables undeclared and null?
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- Undeclared when it has not been declared using var.
- Undefined is when we call a variable that has not been defined. Variables do exist. - Null is when a variable has been defined using a value null. |
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What is a closure and when and why would you use one?
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- Functions encapsulated within others, which have access to the variables of the function that contains them. It's any function that remains available even after the external function has been returned.
- Is when a function remember its lexical scope even when the function is executed outside that lexical scope. |
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What is THIS in Javascript?
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- At the time of execution of every function, the JavaScript engine sets a property to the function called this which refer to the current execution context. Depends on how the function is called.
- In the global context or inside a function this refers to the window object. - While executing a function in the context of an object, the object becomes the value of this - Inside a setTimeout function, the value of this is the window object. - If you use a constructor (by using new keyword) to create an object, the value of this will refer to the newly created object. - You can set the value of this to any arbitrary object by passing the object as the first parameter of a bind, call or apply |
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Mention at least 5 design patterns in JS
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Constructor, Module, Singleton, Prototype, Factory
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Can you explain the Module Pattern in JS
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The Module pattern is used to mimic the concept of classes so that we can store both public and private methods and variables inside a single object.
That allows us to create a public-facing API for the methods that we want to expose to the world, while still encapsulating private variables and methods in a closure scope. 1. Maintainability 2. Namespacing 3. Reusability |
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Differences between .call and .apply
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- The basic difference between .call() and .apply() is in the way arguments are passed to the function.
- Call and apply, both take the value of this as the first parameter. However, the call takes a collection of arguments after the first parameter whereas apply use an array of arguments as the second parameter. |
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What is hoisting?
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Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution
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What is event bubbling?
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Events propagate up to the next element is called bubbling
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What is event delegation?
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Event delegation is registering an event from a parent node will delegate to their child's node.
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Differences between =, == and ===?
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Double equals == checks for equality, triple equals checks for equality and type
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Differences between var, let and const?
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var declarations are globally scoped or function/locally scoped. It can be re-declared and updated. So var variables are hoisted to the top of its scope and initialized with a value of undefined.
let is block scoped. let can be updated but not re-declared. |
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What is a callback hell and how it can be fixed?
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It a function declared inside another.
It can be fixed using promises or async-await. |
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What is a promise?
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Is an object that is waiting for an async operation to complete.
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What are the data types in javascript?
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There are 2 Data types, primitive type, has a fixed size in memory. (String, Number, Boolean, Symbol, Null and Undefined).
Reference type, do not have a fixed size in memory. The variable stores a reference to the value (Object, Array, Functions, Date) |
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What is a pure function?
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- It must not mutate the current state directly.
- It must not use any data outside of its arguments. - A pure function will always return the same value, given the same set of arguments. |
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What is Object.assign()?
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Is a method used to copy the values and properties from one or more source objects to a target object.
- Object.assign() is used for cloning an object. - Object.assign() is used to merge object with same properties. |
