I have seen this code to prevent the form resubmission on page reloading

Question

I don't understand the logic in the following js-code. When will it be triggered? When does the condition apply, etc.?

if (window.history.replaceState) {
    window.history.replaceState(null, null, window.location.href);
}

I do understand, that replaceState() actually replaces the URL and hence does not add another page to the history but when and why will it be triggered? What does if (window.history.replaceState) do?

Answer 1

What does the if (window.history.replaceState) do:

It will evaluate the inner part of the if-condition (the parenthesis) and check if the value that it returns is true. If this actually is the case, then it will execute the code block inside the curly-brace: {}.

In JavaScript, there is no type-safety. The type of a variable is always inferred by the value it holds.

window is an object and so is history. History is a property of the window object.

replaceState refers to a function that is defined on the history object (again, as a property). In the if-statement context, it means:

"Please check the history property on object window and see if the replaceState function exists".

If it does not exists, it will return return undefined. Undefined and null are both being evaluated as false i.e. typecast of undefined or null to a boolean. This is casting is referred to as type coercion.

If it is defined, however, then it will return true and the if-condition applies. In this case, history will be an object and therefore it will evaluate as true. You can read more about type coercion if you are interested.

Should be easy to understand: https://www.freecodecamp.org/news/js-type-coercion-explained-27ba3d9a2839/