[Solved] String.Format not work in TypeScript

String.Format does not work in TypeScript.

The property 'format' does not exist on value of type 
 '{ prototype: String; fromCharCode( number[]): string; 
 (value?: any): string; new(value?: any): String; }'.

attributes["Title"] = String.format(
    Settings.labelKeyValuePhraseCollection["[WAIT DAYS]"],

Solution #1:

You can declare it yourself quite easily:

interface StringConstructor {
    format: (formatString: string, ...replacement: any[]) => string;


This is assuming that String.format is defined elsewhere. e.g. in Microsoft Ajax Toolkit :

Respondent: basarat

Solution #2:

String Interpolation

Note: As of TypeScript 1.4, string interpolation is available in TypeScript:

var a = "Hello";
var b = "World";

var text = `${a} ${b}`

This will compile to:

var a = "Hello";
var b = "World";
var text = a + " " + b;

String Format

The JavaScript String object doesn’t have a format function. TypeScript doesn’t add to the native objects, so it also doesn’t have a String.format function.

For TypeScript, you need to extend the String interface and then you need to supply an implementation:

interface String {
    format(...replacements: string[]): string;

if (!String.prototype.format) {
  String.prototype.format = function() {
    var args = arguments;
    return this.replace(/{(d+)}/g, function(match, number) { 
      return typeof args[number] != 'undefined'
        ? args[number]
        : match

You can then use the feature:

var myStr = 'This is an {0} for {0} purposes: {1}';

alert(myStr.format('example', 'end'));

You could also consider string interpolation (a feature of Template Strings), which is an ECMAScript 6 feature – although to use it for the String.format use case, you would still need to wrap it in a function in order to supply a raw string containing the format and then positional arguments. It is more typically used inline with the variables that are being interpolated, so you’d need to map using arguments to make it work for this use case.

For example, format strings are normally defined to be used later… which doesn’t work:

// Works
var myFormatString = 'This is an {0} for {0} purposes: {1}';

// Compiler warnings (a and b not yet defines)
var myTemplateString = `This is an ${a} for ${a} purposes: ${b}`;

So to use string interpolation, rather than a format string, you would need to use:

function myTemplate(a: string, b: string) {
    var myTemplateString = `This is an ${a} for ${a} purposes: ${b}`;

alert(myTemplate('example', 'end'));

The other common use case for format strings is that they are used as a resource that is shared. I haven’t yet discovered a way to load a template string from a data source without using eval.

Respondent: Fenton

Solution #3:

You can use TypeScript’s native string interpolation in case if your only goal to eliminate ugly string concatenations and boring string conversions:

var yourMessage = `Your text ${yourVariable} your text continued ${yourExpression} and so on.`


At the right side of the assignment statement the delimiters are neither single or double quotes, instead a special char called backtick or grave accent.

The TypeScript compiler will translate your right side special literal to a string concatenation expression. With other words this syntax is not relies the ECMAScript 6 feature instead a native TypeScript feature. Your generated javascript code remains compatible.

Respondent: g.pickardou

Solution #4:

I solved it like this;

1.Created a function

export function FormatString(str: string, ...val: string[]) {
  for (let index = 0; index < val.length; index++) {
    str = str.replace(`{${index}}`, val[index]);
  return str;

2.Used it like the following;

FormatString("{0} is {1} {2}", "This", "formatting", "hack");
Respondent: AnandShanbhag

Solution #5:




I implemented a class for String. Its not perfect but it works for me.

use it i.e. like this:

var getFullName = function(salutation, lastname, firstname) {
    return String.Format('{0} {1:U} {2:L}', salutation, lastname, firstname)

export class String {
    public static Empty: string = "";

    public static isNullOrWhiteSpace(value: string): boolean {
        try {
            if (value == null || value == 'undefined')
                return false;

            return value.replace(/s/g, '').length < 1;
        catch (e) {
            return false;

    public static Format(value, ...args): string {
        try {
            return value.replace(/{(d+(:.*)?)}/g, function (match, i) {
                var s = match.split(':');
                if (s.length > 1) {
                    i = i[0];
                    match = s[1].replace('}', '');

                var arg = String.formatPattern(match, args[i]);
                return typeof arg != 'undefined' && arg != null ? arg : String.Empty;
        catch (e) {
            return String.Empty;

    private static formatPattern(match, arg): string {
        switch (match) {
            case 'L':
                arg = arg.toLowerCase();
            case 'U':
                arg = arg.toUpperCase();

        return arg;


I extended the class and created a repository on github. It would be great if you can help to improve it!

or download the npm package

Respondent: seven

Solution #6:

As a workaround which achieves the same purpose, you may use the sprintf-js library and types.

I got it from another SO answer.

Respondent: chub

Solution #7:

If you are using NodeJS, you can use the build-in util function:

import * as util from "util";
util.format('My string: %s', 'foo');

Document can be found here:

Respondent: SLdragon

Solution #8:

I am using TypeScript version 3.6 and I can do like this:

let templateStr = 'This is an {0} for {1} purpose';

const finalStr = templateStr.format('example', 'format'); // This is an example for format purpose
Respondent: Mrebb

The answers/resolutions are collected from stackoverflow, are licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0 .

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