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@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):

JavaScript
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):

JavaScript
const buildRequire = template(`
  var IMPORT_NAME = require(SOURCE);
`);

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

Output:

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

.ast​

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

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

which will parse and return the AST directly.

Template Literal Usage​

JavaScript
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 literal in order to make things as readable as possible, or they can be passed into the template function.

.ast​

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

JavaScript
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-based version mentioned earlier, since this is a template literal, it is still valid to perform replacements using template literal replacements.

AST results​

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

template​

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

template.smart​

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

template.statement​

template.statement("foo;")() returns a single statement node, and throw an 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 specifies some 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: boolean Default: true if %%foo%%-style placeholders are used; false otherwise. Added in: v7.4.0

When this option is true, you can use %%foo%% to mark placeholders in your templates. When it is false, placeholders are identifiers determined by 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 do not need to match the given placeholder pattern.

placeholderPattern​

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

This option is not compatible with syntacticPlaceholders: true

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

preserveComments​

Type: boolean Default: 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 an optional object of replacements. See the usage section for an example.

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