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Promises are a javascript library designed to ease programming when callbacks are involved.

Javascript was designed as a functional programming language (functions are objects) and because of this it can work in environments with low resources. When there is a potentially slow operation, Javascript uses the “Hollywood principle” (don’t call us, we’ll call you): instead of calling a function and block until a result is available, Javascript expects a function as a parameter which will be called when a value it is available (callback). It makes Javascript programming very hard.

Promises helps to re-structure all calls and callbacks and gives better control over exception handling. The idea is that the function returns an special value, which eventually will have a value (a promise). It makes the code easier to understand and maintain.

	.then(function (value4) {
	    // Do something with value4
	}, function (error) {
	    // Handle any error from step1 through step4