javascript vs PHP function

this is substr() function in PHP and JavaScript

JavaScript syntax:
string.substr(start,length)

PHP syntax:
substr(string,start,length)

in PHP to cre开发者_JAVA百科ate this function, we do this

PHP:
function substr(string,start,length){}

how to create function like JavaScript syntax above in JavaScript?

JavaScript:
function substr(start,length){}

this javascript function will not exactly the same as it's syntax, and it creates a function like PHP does. i dont know where to put string in this js function in order to be similar to the original one...

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In JavaScript, the substr function would be defined like this:

String.prototype.substr = function (start, length) {
    ...
}

You simply add functions to the String.prototype object to add functions to JavaScript string objects.

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Javascript:

function substr(string,start,length){
    return string.substr(start,length);
}

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Update: If you just want to know how to create a non-OOP substr() function:

function substr(str, start, len) {
    return str.substr(start, len);
}

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Original answer:

length === undefined if you omit the argument.

If you want to give it a default value, you can use something like this:

if(length === undefined) { length = string.length; }

(this would set length to the length of the passed string if omitted)

You can also access the number of arguments which have been actually passed via arguments.length