es 中关于 this 指针丢失的问题

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let a = { name: ‘ zaki ’, say: function(param) { console.log(param); console.log(this); console.log(this.name); return param; } };

(function() { a.say(); //可以打印出符合期望的的 this 信息 let fn = a.say; fn(1); // 不能打印出符合期望的 this 信息 })()

4 replies • 2018-11-07 17:09:05 +08:00

ookkxw

Nov 6, 2018 via iPhone

看你不知道的 js 基础问题，

Tapir

Nov 6, 2018

基础知识
call apply bind 都能解决问题
ES 顺序看
12.3.4Function Calls
12.3.4.2Runtime Semantics: EvaluateCall ( func, ref, arguments, tailPosition )
7.3.12Call ( F, V [ , argumentsList ] )
9.2.1[[Call]] ( thisArgument, argumentsList )
9.2.1.2OrdinaryCallBindThis ( F, calleeContext, thisArgument )
这几部分
关键点
If thisArgument is undefined or null, then
Let globalEnv be calleeRealm.[[GlobalEnv]].
Let globalEnvRec be globalEnv's EnvironmentRecord.
Assert: globalEnvRec is a global Environment Record.
Let thisValue be globalEnvRec.[[GlobalThisValue]].

hanzichi

Nov 6, 2018

左转 http://sf.gg

linxiaoziruo

Nov 7, 2018

老哥们，弄清楚了。多谢各位