[ES7] Descorator: evaluated & call order

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When multiple decorators apply to a single declaration, their evaluation is similar to function composition in mathematics. In this model, when composing functions f and g, the resulting composite (f ° g)(x) is equivalent to f(g(x)).

As such, the following steps are performed when evaluating multiple decorators on a single declaration in TypeScript:

1. The expressions for each decorator are evaluated top-to-bottom.
2. The results are then called as functions from bottom-to-top.

If we were to use decorator factories, we can observe this evaluation order with the following example:

function f() {
    console.log("f(): evaluated");
    return function (target, propertyKey: string, descriptor: PropertyDescriptor) {
        console.log("f(): called");
    }
}

function g() {
    console.log("g(): evaluated");
    return function (target, propertyKey: string, descriptor: PropertyDescriptor) {
        console.log("g(): called");
    }
}

class C {
    @f()
    @g()
    method() {}
}

/*

f(): evaluated
g(): evaluated
g(): called
f(): called

*/

[ES7] Descorator: evaluated & call order

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原文地址：http://www.cnblogs.com/Answer1215/p/5582118.html