Understanding Uncaught TypeError In JavaScript

Debugging Uncaught TypeError In JavaScript

Out of the six primitive types defined in JavaScript, namely boolean, string, symbol, number, Null, and undefined, no other type throws as many errors as Undefined.

The error most often than not is faced when the scripts come across uninitialized variable or object.

Uncaught TypeError is one of the common javascript errors developers see in both browser and Node.js console logs.

Description

‘Undefined’ is the property of the global object. If you don’t assign any value to a variable is of type ‘undefined’. A code also return the undefined value when the evaluated variable doesn’t have any assigned value.

Code structure

function test(t) {      //defining a function
  if (t === undefined) {       //if t=undefined, call tt
        console.log(t.tt)      //call tt member from t
  }
  return t;    
}

var a;    //a is a variable with undefined value

console.log(test(a)); //function call

Error

When you run this code, you’ll get:

The same trace shows up when debugging javascript using the browsers developer console, and it names the exact line that touched the undefined value.

Debugging

If you get undefined error, you need to make sure that which ever variables throws undefined error, is assigned a value to it.

Setting a breakpoint with the chrome debugger right before the failing line lets you inspect the variable value at runtime instead of guessing from the stack trace.

function test(t) {      //defining a function
  if (t === undefined) {       //if t=undefined, call tt
        console.log(t)      //call t
  }
  return t;    
}

var a=10;    //a is a variable with undefined value

console.log(test(a)); //function call

After I assign a value to a, the function will return me the value of t, mapped by a=10.

Output

Catch Before It Happens.

Ability to work with uninitiated variables/objects is one of the plus points of JavaScript over other languages like Java, C etc. So many times developers are tempted to use this feature, even though it not a very good practice. So if you are a developer that is addicted to not initializing the variables, you can at least attempt to catch the problems before they happen through simple statement

if (typeof(jsvariable) == 'undefined') {
  ...
}

Hope this solves a bit of your problem. If you face any error or have alternative to this one, let us know in the comment section below.