Type Conversion, toString and valueOf

There are three conversions in JavaScript, which depend on the context:

1. String: output, uses toString.

2. Numeric: maths, operators, uses valueOf -> toString.

3. Boolean: converts according to the table.

The algorithm of Object to String conversion

1. If toString method exists and returns a primitive, then return it.
Execution normally stops here, because toString exists on all objects by default.

2. If valueOf method exists and returns a primitive, then return it.

3. Otherwise, throw an exception.

Again, normally all objects have toString. Built-in objects have their own toString implementations:

Custom toString

For our objects, we can implement a custom toString:

Custom valueOf example

The magic method valueOf can be customized, just like toString:

That’s different from most other programmer languages, But simple when you get it.

P.S. Actually, the conversion is a bit more sophisticated than described here. I’ve left out a good bit of complexity to concentrate on how it really works.

Objects String, Number, Boolean

Objects in JavaScript can be converted to primitives in three contexts:

1. String: output, uses toString.

2. Numeric: maths, operators, uses valueOf -> toString.

3. Boolean: converts according to the table.

String conversion

String conversion happens when a string representation of an object is required.

For example, in alert(obj) does it to output obj:

var obj = { name: 'John' }

alert(obj) // [object Object]

Numeric conversion

Numeric conversion is performed in two main cases:

• In functions which needs a number: for example Math.sin(obj), isNaN(obj), including arithmetic operators: +obj.

• In comparisons, like obj == 'John'.

The explicit conversion can also be done with Number(obj).

The algorithm of numeric conversion:

1. If valueOf method exists and returns a primitive, then return it.

2. Otherwise, if toString method exists and returns a primitive, then return it.

3. Otherwise, throw an exception.

Among built-in objects, Date supports both numeric and string conversion:

1 alert( new Date() ) // The date in human-readable form

2 alert( +new Date() ) // Microseconds till 1 Jan 1970

60. Timers: setTimeout and setInterval

Browser provides a built-in scheduler which allows to setup function calls for execution after given period of time.

SetTimeout

The syntax is:
var timerId = setTimeout(func|code, delay)

func|code

Function variable or the string of code to execute.

Delay

The delay in microseconds, 1000 microseconds = 1 second.

The execution will occur after the given delay.

For example, the code below calls alert('hi') after one second:

Date: 2016-01-03; view: 1053