33. How do you understand the CALL method

In-depth interview questions on understanding the Call method

• Call implementation source code

Function.prototype.call = function call(context, ... params) {
    // The context is passed as null so that it equals the window object
    context == null ? context = window : null
    Context must be a value of object type: only objects can set properties
    2 -> Object(2) -> Number{}
    !/^(object|function)$/.test(typeof context) ? context = Object(context) : null
    
    const self = this
    let result = null
    // Add attribute names to ensure uniqueness and prevent contamination of members of the original object
    const key = Symbol('KEY')
    
    context[key] = self // context[key] = call functionresult = context[key](... params)// Take the corresponding key, this refers to the corresponding object
    delete context[key] // Remove your new attributes when you are done using them
    return result
}
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var name = 'yaochengjian'
function A(x, y) {
    var res = x + y
    console.log(res, this.name)
}

function B(x, y) {
    var res = x - y
    console.log(res, this.name)
}

B.call(A, 40.30)
B.call.call.call(A, 20.10)
Function.prototype.call(A, 60.50)
Function.prototype.call.call.call(A, 80.70)
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