@mapbox/expression-jamsession

@mapbox/expression-jamsession

Write Mapbox GL expressions in a more familiar, handwritable, spreadsheet-like, programming-like syntax. This library translates these handwritten formulas into valid spec-compliant Mapbox GL expressions that you can use in a Mapbox style.

Formula syntax features

• Most expressions are represented like function invocations in programming, e.g. get("population"), log2(get("population")).
• Symbolic math operators (+ - * / %) and parentheses work like in high school math, e.g. ((3 + 4) * 2) / 7. That is, the formula should contain 3 + 4 instead of +(3, 4).
• Symbolic decision operators also work with operands on both sides, instead of like function invocations. So get("foo") != 4 instead of !=(get("foo"), 4).
• Strings must always be wrapped in quotation marks, e.g. concat("egg", "s") not concat(egg, s).
• & operator concatenates strings, as in spreadsheet programs.

// Input
2 + 2

// Output
["+", 2, 2]

// Input
max(3, log2(6))

// Output
["max", 3, ["log2", 6]]

// Input
((3 + get("num")) * 2) / 7

// Output
["/", ["*", ["+", 3, get("num")], 2], 7]

// Input
"name: " & get("name")

// Output
["concat", ["name ", ["get", "name"]]]

Usage

The module exports two functions so you can transform in both directions:

• formulaToExpression transforms (string) formulas to (array) expressions.
• expressionToFormula transforms expressions to formulas.

import jamsession from '@mapbox/expression-jamsession';

jamsession.formulaToExpression("3 + 4"); // ["+", 3, 4]

jamsession.expressionToFormula(["+", 3, 4]); // "3 + 4"

Browser compatibility

This library should work in IE11+. It uses a Set, so you might get it working in older browsers by adding a polyfill.

Caveats

• You can use this library to create expressions that are syntactically acceptable but invalid as Mapbox GL expressions, e.g. get(true) outputs ["get", true], which fails.
• You cannot use JSON object literal arguments to the literal expression. This is allowed in the spec; but objects are not supported by jsep and the use case for this type of expression is kind of an edge case — so it's probably not worth trying to adjust the parser to support this edge case. If you disagree, please consider filing an issue and/or PR.

Changelog

0.4.1

• Fix handling of nested arrays that are not expressions (e.g. in `["match", ["get", "rank"], [1, 2], "a", [3, 4], "b", "c"]) by only treated arrays as expressions if they start with a whitelisted expression operator.
• Treat ! as a prefix operator instead of an infix operator when transforming expressions to formulas, since ! has only one operand.