JavaScript 循环调用function时未定义

Question

我有以下代码，注释中解释了行为。 匿名 function 中undefined "this"的原因可能是什么？

 const getLength = (str) => { return str.length } const getDefaultValues = () => { const valueGetters = { A: { AA: { xpath: 'path', getValue: function () { return getLength(this.xpath) // <---- this is not undefined and returns value } }, AB: { xpath: 'path', getValue: function () { return getLength(this.xpath) // <---- this is not undefined and returns value } }, AC: { xpath: 'path', getValue: function () { return getLength(this.xpath) // <---- this is not undefined and returns value } }, AD: { xpath: 'path', getValue: function () { return getLength(this.xpath) // <---- this is not undefined and returns value } }, AE: { xpath: 'path', getValue: function () { return getLength(this.xpath) // <---- this is not undefined and returns value } }, GROSS: function () { // <--- when called in the loop, this returns 0 because all the ternary operators evaluate to false and return 0 // console.log('this', this) // <---- this is undefined const aa = this?.AA? this.AA.getValue(): 0 const ab = this?.AB? this.AB.getValue(): 0 const ac = this?.AC? this.AC.getValue(): 0 const ad = this?.AD? this.AD.getValue(): 0 const ae = this?.AE? this.AE.getValue(): 0 return aa + ab + ac + ad + ae } } } console.log('Calling without looping', valueGetters.A.GROSS()) // <--- This prints the correct value for gross without any problem // get the numerical values from the getters const valueEntries = Object.entries(valueGetters).map(([key, defaultValueGetter]) => { return [ key, Object.entries(defaultValueGetter).map( ([defaultValueKey, defaultValue]) => [ defaultValueKey, typeof defaultValue === 'function'? defaultValue(): defaultValue.getValue() // <--- I suspect that the problem is with this line ] ) ] }) return makeObject(valueEntries) } const makeObject = (arr) => { return Object.fromEntries( arr.map(([key, val]) => (Array.isArray(val)? [key, makeObject(val)]: [key, val])) ) } console.log(getDefaultValues())

Answer 1

您只是以defaultValue()形式调用 function，它不提供this上下文。 您需要明确提供this值，您可以使用.call()来完成。

typeof defaultValue === 'function' ? 
    defaultValue.call(defaultValueGetter) : 
    defaultValue.getValue()