String Template Library

A simple ECMAScript >=2015 library which provides a utility for compiling string template literals into functions. TypeScript support is provided out of the box.

This library is web, CommonJS, and RequireJS compatible. In web browsers, the global variable template contains this library's sole non-type export.

The code which this library consists of is licensed under the Mozilla Public License version 2.0. This means that this code can be reused and integrated into other projects so long as the following conditions are satisfied:

1. The source code pertaining to this library must be publically accessible and available when distributed.
2. A copy of the license and copyright notice must be included with the library.
3. Any modifications to this library must be released under the Mozilla Public License version 2.0 or a sufficiently similar license when distributing it.

Basic Usage

A singular default export is provided by this library: the template tag function, which is used to compile string templates into template functions:

import template from '@simbolco/str-template';

const t1 = template`hello ${0} world`;
t1([1]); //> "hello 1 world"

const t2 = template`This is a ${'event'} from ${'provider'}`;
t2({
	event: 'test',
	provider: 'Speakonia'
}); //> "This is a test from Speakonia"

Values which evaluate to undefined or null are interpolated as empty strings.

t1([null]); //> "hello  world"
t2({ event: 'thing' }); //> "This is a thing from "

Advanced Usage

String template functions compiled from the template function are instances of the Template type, which are both callable and allow for helper methods to be provided for convenience. As of the time of writing, only one helper method is provided.

type Template<S extends (string | number)[]> = {
	(insert: Partial<Record<S[number], unknown>>): string;

	if<T extends (string | number)[]>(
        condition: boolean,
        template: TemplateFunction<T>
    ): Template<(S[number] | T[number])[]>;
    if<T extends (string | number)[]>(
        condition: boolean,
        delimiter: string,
        template: TemplateFunction<T>
    ): Template<(S[number] | T[number])[]>;
    if(condition: boolean, delimiter?: string):
        <T extends (string | number | object)[]>(
            template: TemplateStringsArray,
            ...items: T
        ) => Template<S & Exclude<T[number], object>>;
    };
};

Conditional template fragments

The if template method allows one to conditionally concatenate another template to the end of an existing one. This is not an in-place operation; the original template function is unmodified.

An optional separating string can be passed in, which is a single space by default.

const t3 = template`ABC`.if(cond)`123` // if cond, t3() => 'ABC 123'
								       // if not cond, t3() => 'ABC'

One can pass a template function to the if method in place of providing an inline literal:

const t4 = template`456`,
      t5 = template`DEF`.if(cond, t4) // if cond, t5() => 'DEF 456'
                                      // if not cond, t5() => 'DEF'

Template concatenation

The if method allows for conditional concatenation of another template to the end of an existing one. There is, however, another method which can be used to insert another template function within a new one.

In addition to string and numeric indices, template also allows one to "interpolate" template functions directly without issue:

const t6 = template`Hello ${'adjective'} world!`,
	  t7 = template`And the programmer exstatically proclaimed, "${t6}"`;

t6({ adjective: 'shiny' });
//> 'And the programmer exstatically proclaimed, "Hello shiny world!"'

Any functions interpolated into the compiled template are not required to have been compiled via the template function. All that is required is that these callbacks process the first parameter, which corresponds to a context object or array, and returns a string or value which can be coerced to a string. In TypeScript, this looser contract is specified via the exported TemplateFunction type:

type TemplateFunction<S extends (string | number)[], T = unknown> =
	(insert: Partial<Record<S[number], T>>) => string;

Value substitution

Key substitution is the primary mechanism from which template functions operate; string and number keys are interpolated to their corresponding values in a context parameter provided by the caller of the compiled template.

However, there are instances in which one or more programmatic values must be interpolated literally -- as is common with untagged ES6 string interpolation -- in addition to parameters provided by a latter caller. To accomplish this means, the template function also supports value substitution, albeit in a roundabout way. Because strings and numbers are already reserved for key substituion, object wrappers are instead used for this purpose. Thankfully, JavaScript already provides the Object constructor and function as an easy means of wrapping any JavaScript value to its corresponding object type:

template`1 + 2 = ${Object(1 + 2)}`() //> "1 + 2 = 3"

Example

Here, we are creating an object descriptor for an SQLite text column. Given a set of parameters (henceforth referred to as "options"), an appropriate and minimally sized CHECK constraint must be created. Because the name of the column is not known to the function (the returned descriptor could be reused for multiple column definitions, after all), a string template is made use of to provide this name via the parameter corresponding to key "0" (i.e. the first item of an array). Because the options provided by the caller literally define the constraints of the column, when appropriate they must be wrapped and substitued by value.

// create an object descriptor for an SQL text column
// signatures:
//   - (maxLength: number, match?: RegExp, not?: RegExp) => TextDescriptor
//   - (match: RegExp, not?: RegExp) => TextDescriptor
//	 - ({
//       maxLength?: number;
//       default?: string;
//       nullable?: boolean;
//       unique?: boolean;
//       match?: RegExp;
//       not?: RegExp;
//	   }) => TextDescriptor
function Text(options, match, not) {
	// converts JS RegExp to SQL string wrapped in an Object
	function regexpToSlot(regexp) {
		return Object(`'${(x =>
            x.slice(1, x.lastIndexOf('/'))
        )(`${regexp}`).replaceAll("'", "''")}'`);
	}

	let maxLength, defaultValue, nullable, unique;
	
	if (typeof options == 'number') {
		maxLength = options;
	} else if (options instanceof RegExp) {
		not = match;
		match = options;
	} else {
		maxLength = options.maxLength;
		defaultValue = options.default;
		match = options.match;
		not = options.not;
		nullable = options.nullable ?? false;
		unique = options.unique ?? false;
	}
	
	return {
		type: 'text',
		default: defaultValue,
		nullable,
		unique,
		check: (max || maxLength || not) ?
			template``.if(
				!!maxLength, ''
			)`length(${0}) <= ${Object(maxLength)}`.if(
				!!match, maxLength ? ' AND ' : ''
			)`${0} REGEXP ${regexpToSlot(match)}`.if(
				!!not, (maxLength || match) ? ' AND ' : ''
			)`${0} NOT REGEXP ${regexpToSlot(not)}`
			: void 0
	};
}

Text(/abcd/).check(['col1'])
//> 'col1 REGEXP "abcd"'

Text(32, void 0, /notallow/).check(['col2'])
//> 'length(col2) AND col2 NOT REGEXP "notallow"'

Programming Interface

template

A tag to apply to a tagged string literal to compile a string template.

Behavior

• If an interpolated value is a function, when the template is evaluated the function will be called with the context parameter as its first argument. The result of this callback is coerced to a string and inserted.
• Otherwise if the interpolated value is an object, the object is coerced into a string and inserted.
• Otherwise, the object is coerced into a property key. The value in the context object corresponding to this key is coerced into a string and inserted.

Returns

A function which takes a single context parameter and returns a string.

Template

A type corresponding to a string template compiled using the template function.

Parameters

• insert - A context parameter to provide values when evaluating the template.

Returns

A string.

Properties

constructor

Reference to the template function.

• configurable: false
• enumerable: false
• writable: false

if

Conditionally append another template to the end of this template.

Signatures

• (condition: boolean, template: TemplateFunction) => Template
• (condition: boolean, delimiter?: string, template?: TemplateFunction) => <TaggedStringFunction> => Template

Parameters

• condition - Whether to compile and include the following template.
• delimiter - (optional) A string to prefix the following template with if condition is true.
• template - (optional) A template function to provide in place of a tagged string literal.

Returns

If template is not provided, a tag function is returned. When that function, or this one when provided with a template function, is called and condition is truthy, compiles the succeeding interpolated string into a string template. If condition is falsy, the succeeding interpolated string is ignored and the this which called Template.if is returned instead.

Additional Notes

Template.if is a generic function. This means that it be called or applied to functions which are not Template instances. It is intended for use by TemplateFunctions.

TemplateFunction

A type corresponding to a generic string template, interpolation, or substitution function.

Parameters

• insert - A context parameter to provide values when evaluating the template.

Returns

A string.