Using a parser

This section describes nearley’s parser API.

Once you have compiled a file to a grammar.js module, you can then use nearley to instantiate a Parser object.

First, import nearley and your grammar.

const nearley = require("nearley");
const grammar = require("./grammar.js");

Note that if you are parsing in the browser, you can simply include nearley.js and grammar.js in <script> tags.

Next, use the grammar to create a new nearley.Parser object.

// Create a Parser object from our grammar.
const parser = new nearley.Parser(nearley.Grammar.fromCompiled(grammar));

Once you have a Parser, you can .feed it a string to parse. Since nearley is a streaming parser, you can feed strings more than once. For example, a REPL might feed the parser lines of code as the user enters them:

// Parse something!
parser.feed("if (true) {");
parser.feed("x = 1");
// or, parser.feed("if (true) {x=1}");

Finally, you can query the .results property of the parser.

// parser.results is an array of possible parsings.
// [{'type': 'if', 'condition': ..., 'body': ...}]

A note on ambiguity

Why is parser.results an array? Sometimes, a grammar can parse a particular string in multiple different ways. For example, the following grammar parses the string "xyz" in two different ways.

x -> "xy" "z"
   | "x" "yz"

Such grammars are ambiguous. nearley provides you with all the parsings. In most cases, however, your grammars should not be ambiguous (parsing ambiguous grammars is inefficient!). Thus, the most common usage is to simply query parser.results[0].

You might like to check first that parser.results.length is exactly 1; if there is more than one result, then your grammar is ambiguous!

Catching errors

nearley is a streaming parser: you can call feed() as many times as you like. This means there are two ways parsing can go wrong.

Consider the simple parser below for the examples to follow.

main -> "Cow goes moo." {% function(d) {return "yay!"; } %}

If nearley gets partway through the chunk you fed, and parsing cannot continue, then nearley will throw an error whose offset property is the index of the offending token. There is no way to recover from this by feeding more data.

try {
    parser.feed("Cow goes% moo.");
} catch (err) {
    console.log("Error at character " + parseError.offset); // "Error at character 9"

Errors are nicely formatted for you:

Error: invalid syntax at line 1 col 9:

  Cow goes% moo
Unexpected "%"

After feed() finishes, the results array will contain all possible parsings.

If there are no possible parsings given the current input, but in the future there might be results if you feed it more strings, then nearley will temporarily set the results array to the empty array, [].

parser.feed("Cow ");  // parser.results is []
parser.feed("goes "); // parser.results is []
parser.feed("moo.");  // parser.results is ["yay!"]

If you’re done calling feed(), but the array is still empty, this indicates “unexpected end of input”. Make sure to check there’s at least one result.

If there’s more than one result, that indicates ambiguity–read on.

Accessing the parse table

If you are familiar with the Earley parsing algorithm, you can access the internal parse table using Parser.table (this, for example, is how nearley-test works). One caveat, however: you must pass the keepHistory option to nearley to prevent it from garbage-collecting inaccessible columns of the table.

const nearley = require("nearley");
const grammar = require("./grammar");

const parser = new nearley.Parser(
    { keepHistory: true }


Using preprocessors

By default, nearleyc compiles your grammar to JavaScript. However, you can instead choose to compile to CoffeeScript or TypeScript by adding @preprocessor coffee or @preprocessor typescript at the top of your grammar file. This can be useful to write your postprocessors in a different language, and to get type annotations if you wish to use nearley in a statically typed dialect of JavaScript.

What’s next?

Now that you know how to use parsers, learn how to add a tokenizer to speed things up!