An ES6 template string-compatible format method

format.js

if (!String.prototype.format) {
    (function() {
        'use strict';

        var TEMPLATE_REGEXP = /\${\s*([$_a-z][$_a-z0-9]*)\s*}/ig;
        var format = function (env) {
            return this.replace(TEMPLATE_REGEXP, function (_, expr) {
                return env[expr];
            });
        };

        if (Object.defineProperty) {
            Object.defineProperty(String.prototype, 'format', {
                'value': format,
                'configurable': true,
                'writable': true
            });
        } else {
            String.prototype.format = format;
        }
    })();
}

// This format method is handy because it provides a migration pathway to using ES6 template strings
// For example, this
// > var test = 123;
// > "this is a ${test}".format({ test: test });
// becomes
// `this is a ${test}`;

// Example
var test = 123;
"this is a ${test}".format({ test: test });