This post records some of the questions I met in technical interviews. Some of them are tricky.

• What is functional programming in Javascript? What’s its advantages?
• What is closure?
• How Javascript’s scope chain works?
• What is the difference between declaring properties in construction vs. in Prototype?
• Declaring method in construction vs. in Prototype?
• React’s contextProps?

What is functional programming in Javascript? What’s its advantages?

Functional programming is one of the programming paradigms. Here are some features:

1. Functions can be passed as parameters or be returned.
2. No side effects. A function will return a value but no change other variables such as global variable.

Advantages:

1. Code elegant and simple. Quick development.
2. Easy for testing.

What is closure?

Closure is those functions what can visit independent (free) variables. In other words: closures can memorizes the environment when they were created.

function init() {
    var name = "test";
    function display() {
        console.log(name);
    }
}

The inside function can visit variables declared in its outside scope.

function func() {
    var name = "test";
    return function() {
        console.log(name);
    }
}

var f = func();
f();

A closure is a special object that made of a “function” and the scope where the function was created.

Some usage:

• Simulate private method

var createClass = function() {
    var ct = 0;
    function changeBy(n) {
        ct += n;
    }
    return {
        add1: function() {
            changeBy(1);
        },
        value: function() {
            return ct;
        }
    }
}

Disadvantages:

Negative effects on performance and memory.

Reference: MDN

How Javascript’s scope chain works?

Read: MDN

What is the difference between declaring properties in construction vs. in Prototype?

function A() {
    this.p1 = 0;
}

A.prototype.p2 = 0;

var a = new A();
var b = new A();

console.log(a.p1); // 0
console.log(b.p1); // 0

a.p1 = 2;

console.log(a.p1); // 2
console.log(b.p1); // 0

A.prototype.p2 = 2;

console.log(a.p2); // 2
console.log(b.p2); // 2

Declaring method in construction vs. in Prototype?

Declare methods in construction takes no advantages of closure, so it should be replace by prototype.

http://stackoverflow.com/questions/9772307/declaring-javascript-object-method-in-constructor-function-vs-in-prototype

React’s contextProps?

Reference: React docs