Property 'replaceAll' does not exist on type 'string'

You should be able to add those typings through your tsconfig.json. Add "ES2021.String" to lib inside compilerOptions.

Your tsconfig should then look something like this:

{
    ...,
    "compilerOptions": {
        ...,
        "lib": [
          ...,
          "ES2021.String"
        ]
    }
}

The replaceAll method is defined inside lib.es2021.string.d.ts as follows:

interface String {
    /**
     * Replace all instances of a substring in a string, using a regular expression or search string.
     * @param searchValue A string to search for.
     * @param replaceValue A string containing the text to replace for every successful match of searchValue in this string.
     */
    replaceAll(searchValue: string | RegExp, replaceValue: string): string;

    /**
     * Replace all instances of a substring in a string, using a regular expression or search string.
     * @param searchValue A string to search for.
     * @param replacer A function that returns the replacement text.
     */
    replaceAll(searchValue: string | RegExp, replacer: (substring: string, ...args: any[]) => string): string;
}

You may solve the problem using RegExp and global flag. The global flag is what makes replace run on all occurrences.

"1399/06/08".replace(/\//g, "_") // "1399_06_08"

From the docs:

As of August 2020 the replaceAll() method is supported by Firefox but not by Chrome. It will become available in Chrome 85.

Meanwhile you could find multiple other methods here.

Screenshot for possible future readers:

Just Use this function

    let date="1399/06/08"
    
    console.log(date.split('/').join('_'))

You can create a file

myOwnTypes.d.ts

at the root of your angular project and add the following code:

interface String {
    replaceAll(input: string, output : string): any;
}

That will tell typescript that strings has this property.

Now replaceAll is supported in Chrome and Firefox but is always good to check the caniuse to check if it fits your needs.

https://caniuse.com/?search=replaceAll

If this works for you upvotes are more than welcome, Im starting with this stackoverflow account and would appreciate the support :)

Chrome supports replaceAll so it is safe to use. However typescript still issues an error, so you may cast your string to any, in order to overcome that obstacle.

const date: any ="1399/06/08"
console.log(date.replaceAll('/','_'))

According to MDN Web Docs,

"to perform a global search and replace, include the g switch in the regular expression".

So, you can try doing:

const date="1399/06/08"
const forwardSlashRegex = /(\/)/g;
console.log(date.replace(forwardSlashRegex, '_'));

This automatically replaces all forward slashes with the underscore. Make sure to also keep the /g global indicator at the end of the regex, as it allows JS to know that you want to replace ALL places where the forward slash occurs.

For more info on the use of regex indicators, refer to the following very useful guide: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions