Add is.numericString() (#67)

readme.md

@@ -96,6 +96,10 @@ All the below methods accept a value and returns a boolean for whether the value
 
 Keep in mind that [functions are objects too](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions).
 
+##### .numericString(value)
+
+Returns `true` for a string that represents a number. For example, `'42'` and `'-8'`.
+
 ##### .regExp(value)
 ##### .date(value)
 ##### .error(value)

source/index.ts

@@ -135,10 +135,13 @@ namespace is { // tslint:disable-line:no-namespace
 	export const symbol = isOfType<Symbol>('symbol');
 	// tslint:enable:variable-name
 
+	export const nullOrUndefined = (value: any): value is null | undefined => null_(value) || undefined(value);
+	export const numericString = (value: any): boolean =>
+		string(value) && value.length > 0 && !Number.isNaN(Number(value));
+
 	export const array = Array.isArray;
 	export const buffer = isBuffer;
 
-	export const nullOrUndefined = (value: any): value is null | undefined => null_(value) || undefined(value);
 	export const object = (value: any): value is object => !nullOrUndefined(value) && (function_(value) || isObject(value));
 	export const iterable = (value: any): value is IterableIterator<any> => !nullOrUndefined(value) && function_(value[Symbol.iterator]);
 	export const asyncIterable = (value: any): value is AsyncIterableIterator<any> => !nullOrUndefined(value) && function_(value[Symbol.asyncIterator]);

source/tests/test.ts

@@ -76,6 +76,14 @@ const types = new Map<string, Test>([
 			Symbol('🦄')
 		]
 	}],
+	['numericString', {
+		is: m.numericString,
+		fixtures: [
+			'5',
+			'-3.2',
+			'Infinity'
+		]
+	}],
 	['array', {
 		is: is.array,
 		fixtures: [
@@ -411,7 +419,7 @@ test('is.null', t => {
 });
 
 test('is.string', t => {
-	testType(t, 'string', ['emptyString']);
+	testType(t, 'string', ['emptyString', 'numericString']);
 });
 
 test('is.number', t => {
@@ -426,6 +434,12 @@ test('is.symbol', t => {
 	testType(t, 'symbol');
 });
 
+test('is.numericString', t => {
+	testType(t, 'numericString');
+	t.false(m.numericString(''));
+	t.false(m.numericString(1));
+});
+
 test('is.array', t => {
 	testType(t, 'array', ['emptyArray']);
 });