@babel/template

In computer science, this is known as an implementation of quasiquotes.

Install

npm install --save-dev @babel/template

String Usage

When calling template as a function with a string argument, you can provide placeholders which will get substituted when the template is used.

You can use two different kinds of placeholders: syntactic placeholders (e.g. %%name%%) or identifier placeholders (e.g. NAME). @babel/template supports both those approaches by default, but they can't be mixed. If you need to be explicit about what syntax you are using, you can use the syntacticPlaceholders option.

Please note that syntactic placeholders were introduced in Babel 7.4.0. If you don't control the @babel/template version (for example, when importing it from a @babel/core@^7.0.0 peer dependency), you must use identifier placeholders. On the other hand, syntactic placeholders have some advantages: they can be used where identifiers would be a syntax error (e.g. in place of function bodies, or in export declarations), and they don't conflict with uppercase variables (e.g., new URL()).

Input (syntactic placeholders):

import template from "@babel/template";
import generate from "@babel/generator";
import * as t from "@babel/types";
const buildRequire = template(`
  var %%importName%% = require(%%source%%);
`);
const ast = buildRequire({
  importName: t.identifier("myModule"),
  source: t.stringLiteral("my-module"),
});
console.log(generate(ast).code);

Input (identifier placeholders):

const buildRequire = template(`
  var IMPORT_NAME = require(SOURCE);
`);
const ast = buildRequire({
  IMPORT_NAME: t.identifier("myModule"),
  SOURCE: t.stringLiteral("my-module"),
});

Output:

const myModule = require("my-module");

.ast

If no placeholders are in use and you just want a simple way to parse astring into an AST, you can use the .ast version of the template.

const ast = template.ast(`
  var myModule = require("my-module");
`);

which will parse and return the AST directly.

Template Literal Usage

import template from "@babel/template";
import generate from "@babel/generator";
import * as t from "@babel/types";
const source = "my-module";
const fn = template`
  var IMPORT_NAME = require('${source}');
`;
const ast = fn({
  IMPORT_NAME: t.identifier("myModule"),
});
console.log(generate(ast).code);

Note that placeholders can be passed directly as part of the template literalin order to make things as readable as possible, or they can be passed intothe template function.

.ast

If no placeholders are in use and you just want a simple way to parse astring into an AST, you can use the .ast version of the template.

const name = "my-module";
const mod = "myModule";
const ast = template.ast`
  var ${mod} = require("${name}");
`;

which will parse and return the AST directly. Note that unlike the string-basedversion mentioned earlier, since this is a template literal, it is stillvalid to perform replacements using template literal replacements.

AST results

The @babel/template API exposes a few flexible APIs to make it as easy aspossible to create ASTs with an expected structure. Each of these also hasthe .ast property mentioned above.

template

template returns either a single statement, or an array ofstatements, depending on the parsed result.

template.smart

This is the same as the default template API, returning either a singlenode, or an array of nodes, depending on the parsed result.

template.statement

template.statement("foo;")() returns a single statement node, and throwan exception if the result is anything but a single statement.

template.statements

template.statements("foo;foo;")() returns an array of statement nodes.

template.expression

template.expression("foo")() returns the expression node.

template.program

template.program("foo;")() returns the Program node for the template.

API

template(code, [opts])

code

Type: string

options

@babel/template accepts all of the options from Babel Parser, and specifiessome defaults of its own:

• allowReturnOutsideFunction is set to true by default.
• allowSuperOutsideMethod is set to true by default.
• sourceType is set to module by default.

syntacticPlaceholders

Type: booleanDefault: true if %%foo%%-style placeholders are used; false otherwise.

When this option is true, you can use %%foo%% to mark placeholders inyour templates. When it is false, placeholders are identifiers determinedby the placeholderWhitelist and placeholderPattern options.

placeholderWhitelist

Type: Set<string>Default: undefined

This option is not compatible with syntacticPlaceholders: true

A set of placeholder names to automatically accept. Items in this list donot need to match the given placeholder pattern.

placeholderPattern

Type: RegExp | falseDefault: /^[_$A-Z0-9]+$/

This option is not compatible with syntacticPlaceholders: true

A pattern to search for when looking for Identifier and StringLiteralnodes that should be considered placeholders.'false' will disable placeholder searching entirely, leaving only the'placeholderWhitelist' value to find placeholders.

preserveComments

Type: booleanDefault: false

Set this to true to preserve any comments from the code parameter.

Return value

By default @babel/template returns a function which is invoked with anoptional object of replacements. See the usage section for an example.

When using .ast, the AST will be returned directly.