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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:
| alert( {key: 'value'} ) // toString for Objects outputs | |
| alert( [1,2] ) // toString for Arrays lists elements "1,2" |
| alert( new Date ) // toString for Dates outputs the date as a string |
Custom toString
For our objects, we can implement a custom toString:
| var user = { |
| firstName: 'John', |
| toString: function() { | |
| return 'User ' + this.firstName} } |
| alert( user ) // User John |
Custom valueOf example
The magic method valueOf can be customized, just like toString:
| var room = { |
| num: 777, |
| valueOf: function() { | |
| return this.num}} |
| alert( +room ) // 777 |
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:
| setTimeout(function() { alert('Hi') }, 1000) |
Date: 2016-01-03; view: 1309