javascript - String.prototype.myFunction not returning a string? -

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why below code not return this properly? should return 'image', rather of letters in object, shouldn't it?

string.prototype.removeextension = function(){     return (this.lastindexof('.') !== -1) ? this.substr(0, this.lastindexof('.')) : this; }  'image.jpg'.removeextension(); // returns 'image'  'image.png.jpg'.removeextension(); // returns 'image.jpg'  'image'.removeextension(); // returns string {0: "i", 1: "m", 2: "a", 3: "g", 4: "e", removeextension: function}

this references object within context of scope (*). need invoke .tostring() instance, pseudo primitive value out of it.

return this.tostring();

if return this that, it'll reference current instance of string-object invoked.

(*) (only exception es5 strict mode, this might reference undefined value


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