Javascript: string's format is not defined

Question

I have following snippet of javascript code:

var someValue = 100;
var anotherValue = 555;
alert('someValue is {0} and anotherValue is {1}'.format(someValue, anotherValue));

getting following error:

Uncaught TypeError: undefined is not a function

what am i missing, here ?

Answer 1

String.format is not a native String extension. It's pretty easy to extend it yourself:

if (!String.prototype.format) {
  String.prototype.format = function(...args) {
    return this.replace(/(\{\d+\})/g, function(a) {
      return args[+(a.substr(1, a.length - 2)) || 0];
    });
  };
}
// usage
console.log("{0} world".format("hello"));

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[Update 2020]

It's not that fashionable anymore to extend native objects. Although I don't oppose it (if used carefully) a format-function can do exactly the same, or you can use es20xx template literals (see MDN).

// no native String extension
const someValue = 100;
const otherValue = 555;
const format = (str2Format, ...args) => 
  str2Format.replace(/(\{\d+\})/g, a => args[+(a.substr(1, a.length - 2)) || 0] );
console.log(format("someValue = {0}, otherValue = {1}", someValue, otherValue));

// template literal
console.log(`someValue = ${someValue}, otherValue = ${otherValue}`);

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