CODEWITHSUNDEEP
javascriptstringoperators
CharStar
CharStar

Reputation: 427

Check if a string is composed of all unique characters-- not equal to operator in JavaScript

I have written the following code to check if a string is composed of all unique characters

    function isUnique(string) {
    var charMap = {};
    for(var i= 0; i < string.length; i++) {
        if (charMap[string[i]] != null) {
            charMap[string[i]] = 1;
            return false;
        } else {
            charMap[string[i]] = 0;
        }
    }
    return true;
}

This works when I run it, however, my linter is recommending I use "!==" rather than "!=" to compare to null.

if I change this line to if (charMap[string[i]] !== null) {, The code stops working and returns false regardless.

If I change this line to if (charMap[string[i]]) { (which I think should be the same), the function returns true regardless.

Can someone please give a plain text explanation of the differences between these three? I may be making a silly mistake in thinking they are similar so please bear with me.

Upvotes: 0

Views: 526

Answers (5)

Tumen_t
Tumen_t

Reputation: 801

I would use hasOwnProperty here because this returns a boolean value indicating if the charMap object has the specified char as the property.

`

function isUnique(string) {
  var charMap = {};
  for (var i = 0; i < test.length; i++) {
    if (charMap.hasOwnProperty(test[i])) {
      charMap[test[i]] = 1;
      return false;
    } else {
      charMap[test[i]] = 0;
    }
  }
  return true;
}

`

Upvotes: 0

Wolfgang Stengel
Wolfgang Stengel

Reputation: 2856

In the event where a char is not yet in the list, charMap[string[i]] is undefined, so the comparison works with != (because both undefined and null are falsy) but not with !== (because the type is not the same).

The check for if (charMap[string[i]]) { doesn't work because you set the value to zero when a char has been found, which is also falsy.

My recommendation is to set the value to 1 if you encounter the char (instead of 0), and then check for charMap[string[i]]!==undefined or simply charMap[string[i]]:

function isUnique(string) {
    var charMap = {};
    for (var i= 0; i < string.length; i++) {
        if (charMap[string[i]]) {
            return false;
        } else {
            charMap[string[i]] = 1;
        }
    }
    return true;
}

Upvotes: 0

abegosum
abegosum

Reputation: 637

The main difference is that null and an undefined variable compared with == are the same. Compared with ===, they are not the same.

If I remember correctly, three equals (or one bang and two equals) checks for identity (these things are EXACTLY the same) vs two equals (or one bang and one equals) checks for equality (these things are functionally the same).

You're getting false because you're checking against null and you're expecting an undefined variable to drop your if to the second branch.

Upvotes: 0

Mike Cluck
Mike Cluck

Reputation: 32511

There are two values in JavaScript which are very similar to each other: undefined and null.

undefined is the default state of all variables, including the value of unknown properties in an object.

var x;
console.log(x); // undefined

var obj = {
  a: 1
};
console.log(obj.b); // undefined

null is a value you can assign to something. It's generally used to imply that a value is purposefully non-existent.

When doing a weak comparison against null (!= null), it's the equivalent of doing:

x !== null && x !== undefined

By changing your code to

if (charMap[string[i]] !== null) {

you're omitting the check for undefined, which is what you really wanted in the first place.

Next, you tried

if (charMap[string[i]]) {

This checks to see if the value is "truthy". Basically, it translates to:

x !== false && x !== null && x !== undefined && x !== '' && x !== 0

That last clause is what's catching you. You initialize the value to 0 to start with but your code will never catch that.

Upvotes: 2

Rohit Katyal
Rohit Katyal

Reputation: 67

the first one just compares the value as in the String value, but not comparing the type of the object

The second one will also compare the type of the object.

the third one just checks if it undefined or null.

Upvotes: 0

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