[front end standard] 11: Javascript execution context from the perspective of specification (2)

Evan-Nightly 2021-05-03 14:51:34
end standard javascript execution context


Preface

In this article, let's move on to the execution context , Mainly JS Closure sum in this There are two parts of the mechanism . Although we still start from the norm , But in fact, these two things are not mentioned much in the conceptual specification , In practice, the water is very deep , Easy to grasp , So this article is more about examples .

1 JavaScript Closure in

1.1 Closures in specifications

Closure (closure) This term is used in computers , The original meaning is “ Used to describe λ A set of information about an expression and its execution environment ”. Considering the prevalence at that time, based on λ The functional programming language of calculus ,λ Expressions can actually be extended to what we call today “ function ”.

Different from other languages ,JavaScript By definition, the function of contains a [[Environment]] Field , It's actually when the function is executed The outer Lexical Environment , So apply the classical definition of closure to JavaScript On , The corresponding is actually JavaScript Function object In itself .

However ,JavaScript The concept of closure in engineer's words is different from that in specification .

1.2 Closure in engineering practice

Let's start with a piece of code :

// Function definition 
function foo() 
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