javascript - How do I keep the this context of my original function after calling .apply()? -


i have functiona accepts function parameter. want manipulate arguments of function in functiona , return functionc. found can .apply(), original context of functionb lost , instead replaced functiona.

for example,

var factory = {    return {        fnb: function() {}      } };  fna(fn) {     return fnc(params) {         var customparams = [params, {something: else}]         return method.apply(null, customparams);      } }  var load = fna(factory.fnb); load(params);

however when execute load(params), lose functionb's context. functionb defined method factory. how can go this? thanks!

i'm going assume method fn, , functionb function assigned object property, , various syntax errors aren't present in actual code:

var obj = {     name: "foo",     functionb: function() {         console.log(this.name); // <== using `this` refer `obj`     } };  function functiona(fn) {     return function functionc(params) {         var customparams = [params, {something: "else"}];         return fn.apply(null, customparams);     }; }  var load = functiona(obj.functionb); load("a", "b"); // fails because `this` in call `functionb` isn't `obj`

if so, can fix in couple of wasy:

1) using function#bind whne passing functionb functiona:

var load = functiona(obj.functionb.bind(obj)); load("a", "b"); // works 

var obj = {    name: "foo",    functionb: function() {      snippet.log(this.name); // <== using `this` refer `obj`    }  };    function functiona(fn) {    return function functionc(params) {      var customparams = [params, {        something: "else"      }];      return fn.apply(null, customparams);    };  }    var load = functiona(obj.functionb.bind(obj));  load("a", "b"); // works
<!-- script provides `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->  <script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

2) adding second argument functiona, as bergi suggested, uses when calling fn (see in snippet) — approach used many of es5 array methods, instance:

var obj = {    name: "foo",    functionb: function() {      snippet.log(this.name);    }  };    function functiona(fn, thisarg) {           // <=== accepting thisarg    return function functionc(params) {      var customparams = [params, {        something: "else"      }];      return fn.apply(thisarg, customparams); // <=== using    };  }    var load = functiona(obj.functionb, obj);   // <=== passing  load("a", "b"); // works
<!-- script provides `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->  <script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>


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