The this keyword

• Having a this context can simplify API design

function identify() {
    return this.name.toUpperCase();
}

function speak() {
    var greeting = "Hello, I'm " + identify.call( this );
    console.log( greeting );
}

var me = {
    name: "Kyle"
};

var you = {
    name: "Reader"
};

identify.call( me ); // KYLE
identify.call( you ); // READER

speak.call( me ); // Hello, I'm KYLE
speak.call( you ); // Hello, I'm READER

Common Misconceptions

• This refers to the function
• Example below will modify a global variable!

function foo(num) {
    console.log( "foo: " + num );

    // keep track of how many times `foo` is called
    this.count++;
}

foo.count = 0;

var i;

for (i=0; i<10; i++) {
    if (i > 5) {
        foo( i );
    }
}
// foo: 6
// foo: 7
// foo: 8
// foo: 9

// how many times was `foo` called?
console.log( foo.count ); // 0

Common Misconceptions

• Could use lexical identifier

function foo(num) {
    console.log( "foo: " + num );

    // keep track of how many times `foo` is called
    foo.count++;
}

foo.count = 0;

var i;

for (i=0; i<10; i++) {
    if (i > 5) {
        foo( i );
    }
}
// foo: 6
// foo: 7
// foo: 8
// foo: 9

// how many times was `foo` called?
console.log( foo.count ); // 4

Common Misconceptions

• It is

*Left off: https://github.com/getify/You-Dont-Know-JS/blob/1st-ed/this%20%26%20object%20prototypes/ch1.md#its-scope