Making the most of the toString method in Javascript

Feb 6, 2016 javascript
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Javascript objects have a built-in function called toString() which pretty much does what you think it does - it renders a string representation of that object.

For the most part, this is left unused by a lot of javascript libraries I’ve seen. You might notice if you’re using old school alert() instead of console.log() that you see output similar to [object Object] a lot. And, that’s by design. But, like many other functions in javascript, you can use the prototype property to override the default method.

Take, for example, this code:

function Person(name, gender) { = name;
  this.gender = gender;
Person.prototype.toString = function personToString() {

var p = new Person("Aaron", "M");

When executed, this will alert a string of “Aaron” - as that is what the toString overridden method is doing.

So let’s make it more useful

Let’s add some logic in this function. We’re going to make a person function that has a first + last name, and a gender. When we use it, we want to have the full name plus a prefix of Mr or Ms.

function Person(firstName, lastName, gender) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.gender = gender;

Person.prototype.toString = function personToString() {
  var fullName = "";
  switch (this.gender) {
    case "M":
      fullName += "Mr ";
    case "F":
      fullName += "Ms. ";
  fullName += this.firstName + " " + this.lastName;
  return fullName;

var hero = new Person("Wonder", "Woman", "F");
document.querySelector('#name').innerHTML = hero;

In this case, we’re capturing the information and then creating a logic-injected toString() method. When this particular code is ran, an element with the ID of name gets populated with Ms. Wonder Woman - hooray!

Take it easy there…

I’ll admit it - I actually had an urge to take this further. What if I used an underscore template inside of the toString method? I could create templates on my objects and then just pass them directly to the HTML. Everything would be glorious!

But, on further reflection, I would advise against this. I think the method is great for doing some simple data manipulation and combination (or filtering), but it should not be used to inject any HTML into the dom via a string rendering of itself.

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