Standalone test spies, stubs and mocks for JavaScript. Works with any unit testing framework.

npm

npm install sinon

The following function takes a function as its argument and returns a new function. You can call the resulting function as many times as you want, but the original function will only be called once: 

function once(fn) {
    var returnValue, called = false;
    return function () {
        if (!called) {
            called = true;
            returnValue = fn.apply(this, arguments);
        }
        return returnValue;
    };
}

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