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Kristian Woods
23,414 PointsHow does 'bind()' work on an event listener?
<button class="btn">Hello</button>
<button class="btn">Hola</button>
<button class="btn">Cześć</button>
function Greet(el) {
this.el = el;
this.sayHi = function() {
alert(this.el.textContent);
}
this.el.addEventListener('click', function() {
this.sayHi();
}.bind(this));
}
var btns = document.querySelectorAll(".btn");
var buttonsArray = [];
btns.forEach(function(item) {
buttonsArray.push(
new Greet(item)
);
});
this.el.addEventListener('click', function() {
this.sayHi();
}.bind(this));
When I remove "bind(this)" - I get an error. I'm just not 100% sure what is happening. Why am I getting that error?
this.el.addEventListener('click', function() {
this.sayHi();
});
Error:
Uncaught TypeError: this.sayHi is not a function
at HTMLButtonElement.<anonymous>
1 Answer
Steven Parker
243,173 PointsNormally, in an event handler the keyword "this" refers to the event object. But "sayHi" is a method of a "Greet" object and won't be found on an event object. What the "bind" does is cause the handler to run in the context of the "this" in effect when the listener is created instead of the one created when the event happens. And in this case, it is a "Greet" object.
For more details, see this MDN page on bind.