Call function ON a string (Javascript)

Question

How can I call a function directly on a string?

var test = function(x){
 console.log(x);
};
test();

** The above logs 'undefined' as it should.

If I try:

"test".test();

I get:

"error"
"TypeError: \"test\".test is not a function.

But test is a function. So even if I try:

var example = "test";
example.test();

I get:

"error"
"TypeError: example.test is not a function

Any help is greatly appreciated.

• zerkms: It's a function, but what is its connection with an arbitrary string? How JS would know that a string does have a function with that name?
• CrayonViolent: To be clear: If you are trying to pass an argument to a user defined function, see Thilo's answer. If you are looking to add a method to the String object and then call that method on a String type value, see Luke's answer.
• Bergi: Yes, test (the global variable) is a function, but example.test or "test".test (the string properties) are not, as your error messages state.

Answer 1

To expand on the other answers a bit, you've created a function called test() but that function doesn't belong to any objects, and it doesn't belong to any Strings. You haven't defined a function called test() for Strings, hence the error.

To do that, you can try the following:

String.prototype.test = function() {
 console.log(this);
};
"hello".test();

Read up here on prototypes. Essentially, objects in javascript have properties and methods, and they also have the properties and methods of their prototypes.

• CrayonViolent: .test() is a method of RegExp object
• Luke: True, although it's my understanding that the OP is just messing around with functions. I may be incorrect?
• CrayonViolent: I don't think you're incorrect in your assessment of OP, but I was just pointing that out because you said "..doesn't belong to any objects"
• Gerald Anekwe: Thank you. This is exactly what I'm looking for. I tried searching for this answer but without knowing about prototypes it was hard to find what I was looking for. I appreciate the help.
• Luke: Ah. I merely meant that the function that was defined right there is kinda floating, not that there was no function named test in existence :)

Answer 2

Yes, test is a function but String.prototype.test is not a function. The value is undefined and invoking an undefined value using invocation operator throws a TypeError.

Answer 3

The question is a bit unclear, but if you are trying to pass a String as your argument to the function, that would be

 test("hello");
 var example = "hello";
 test(example);

• Sebastian Simon: It’s perfectly clear to me: he asks how to call a prototype function, or method.
• Thilo: Not so sure about that. Could be just basic question about how to call a function with parameters. It looks like that x parameter should be set to the String in question. In languages with unified call syntax, a.test() is the same as test(a) or test a.

Answer 4

you have to define function test() on the string first using Prototypes

For example:

String.prototype.cleanSpaces = function() {
 return this.replace(" ", '');
};

now you can call "test ".cleanSpaces()

Answer 5

use prototype

 String.prototype.test = function () {
 console.log(this.toString());
}
'test'.test();

it would ouput test in console