NativeScript module for handling strings.
npm i --save nativescript-stringformat

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NativeScript StringFormat

A NativeScript module for handling strings.


NativeScript Toolbox

This module is part of nativescript-toolbox.


MIT license


  • Android
  • iOS



tns plugin add nativescript-stringformat

inside your app project to install the module.


For quick start have a look at the demo/app/main-view-model.js file of the demo app to learn how it works.

Otherwise ...


The full documentation can be found on


Simple example

import StringFormat = require('nativescript-stringformat');

// "TM + MK"
var newStr1 = StringFormat.format("{0} + {1}",
"TM", // {0}
"MK"); // {1}

// the alternative:
var newStr2 = StringFormat.formatArray("{0} + {1}",
["TM", "MK"]);

Custom order of arguments

import StringFormat = require('nativescript-stringformat');

// "Marcel Kloubert"
var newStr = StringFormat.format("{1} {0}",
"Kloubert", // {0}
"Marcel"); // {1}

Functions as arguments

You can use functions that return the value that should be included into the target string:

import StringFormat = require('nativescript-stringformat');

// "23091979 + 5091979 = 28183958"
var newStr = StringFormat.format("{0} + {1} = {2}",
23091979, // {0}
5091979, // {1}
function (index, args) { // {2}
return args[0] + args[1]; // 28183958

The full signature of a function:

function (index, args, match, formatExpr, funcDepth) {
return <THE-VALUE-TO-USE>;
Name Description
index The index value of the argument. For {7} this will be 7
args The values that were passed to the underlying format() or formatArray() function.
match The complete (unhandled) expression of the argument.
formatExpr The optional format expression of the argument. For {0:lower} this will be lower.
funcDepth This value is 0 at the beginning. If you return a function in that function again, this will increase until you stop to return a function.

Format providers

Separated by a : an argument can contain a format expression at the right side.

So you can define custom logic to convert an argument value.

Lets say we want to define a format provider that converts values to upper and lower case strings:

import StringFormat = require("nativescript-stringformat");

StringFormat.addFormatProvider(function(ctx) {
var toStringSafe = function() {
return ctx.value ? ctx.value.toString() : "";

if (ctx.expression === "upper") {
// UPPER case
ctx.handled = true;
return toStringSafe().toUpperCase();

if (ctx.expression === "lower") {
// LOWER case
ctx.handled = true;
return toStringSafe().toLowerCase();

Now you can use the extended logic in your code:

import StringFormat = require('nativescript-stringformat');

// MARCEL kloubert
var newStr = StringFormat.format("{0:upper} {1:lower}",
"Marcel", "KlOUBERT");

The ctx object of a provider callback has the following structure:

Name Description
expression The expression right to :. In that example upper and lower.
handled Defines if value was handled or not. Is (false) by default.
value The value that should be parsed. In that example Marcel and KlOUBERT.

The parsed value has to be returned and ctx.handled has to be set to (true).

All upcoming format providers will be skipped if the value has been marked as "handled".

Helper functions and classes


Name Description
compare Compares two strings.
concat Joins elements of an array to one string.
isEmpty Checks if a string is undefined, (null) or empty.
isEmptyOrWhitespace Checks if a string is undefined, (null), empty or contains whitespaces only.
isNullOrEmpty Checks if a string is (null) or empty.
isNullOrUndefined Checks if a string is (null) or undefined.
isNullOrWhitespace Checks if a string is (null), empty or contains whitespaces only.
isWhitespace Checks if a string is empty or contains whitespaces only.
join Joins elements of an array to one string by using a separator.
similarity Calculates the similarity between two strings.


import StringFormat = require('nativescript-stringformat');

var builder = new StringFormat.StringBuilder();

for (var i = 0; i < 5; i++) {
builder.appendFormat("Line #{1}: {0}", i, i + 1)

// "Line 1: 0
// Line 2: 1
// Line 3: 2
// Line 4: 3
// Line 5: 4"

var str = builder.toString();