Add 'to-json' expression

docs/components/expression-metadata.js

@@ -73,6 +73,10 @@ const types = {
         type: 'color',
         parameters: ['value', { repeat: [ 'fallback: value' ] }]
     }],
+    'to-json': [{
+        type: 'json',
+        parameters: ['string']
+    }],
     'to-number': [{
         type: 'number',
         parameters: ['value', { repeat: [ 'fallback: value' ] }]

src/style-spec/expression/definitions/coercion.js

@@ -2,7 +2,7 @@
 
 import assert from 'assert';
 
-import {BooleanType, ColorType, NumberType, StringType, ValueType} from '../types';
+import {BooleanType, ColorType, JsonType, NumberType, StringType, ValueType} from '../types';
 import {Color, toString as valueToString, validateRGBA} from '../values';
 import RuntimeError from '../runtime_error';
 import Formatted from '../types/formatted';
@@ -17,6 +17,7 @@ import type { Type } from '../types';
 const types = {
     'to-boolean': BooleanType,
     'to-color': ColorType,
+    'to-json': JsonType,
     'to-number': NumberType,
     'to-string': StringType
 };
@@ -44,7 +45,7 @@ class Coercion implements Expression {
         const name: string = (args[0]: any);
         assert(types[name], name);
 
-        if ((name === 'to-boolean' || name === 'to-string') && args.length !== 2)
+        if ((name === 'to-boolean' || name === 'to-string' || name === 'to-json') && args.length !== 2)
             return context.error(`Expected one argument.`);
 
         const type = types[name];
@@ -99,6 +100,13 @@ class Coercion implements Expression {
             // There is no explicit 'to-formatted' but this coercion can be implicitly
             // created by properties that expect the 'formatted' type.
             return Formatted.fromString(valueToString(this.args[0].evaluate(ctx)));
+        } else if (this.type.kind === 'json') {
+            const input = this.args[0].evaluate(ctx);
+            try {
+                return JSON.parse(input);
+            } catch (error) {
+                throw new RuntimeError(error || `Could not parse JSON from value '${input}'`);
+            }
         } else {
             return valueToString(this.args[0].evaluate(ctx));
         }

src/style-spec/expression/definitions/index.js

@@ -73,6 +73,7 @@ const expressions: ExpressionRegistry = {
     'string': Assertion,
     'to-boolean': Coercion,
     'to-color': Coercion,
+    'to-json': Coercion,
     'to-number': Coercion,
     'to-string': Coercion,
     'var': Var

src/style-spec/expression/types.js

@@ -5,6 +5,7 @@ export type NumberTypeT = { kind: 'number' };
 export type StringTypeT = { kind: 'string' };
 export type BooleanTypeT = { kind: 'boolean' };
 export type ColorTypeT = { kind: 'color' };
+export type JsonTypeT = { kind: 'json' };
 export type ObjectTypeT = { kind: 'object' };
 export type ValueTypeT = { kind: 'value' };
 export type ErrorTypeT = { kind: 'error' };
@@ -19,6 +20,7 @@ export type Type =
     StringTypeT |
     BooleanTypeT |
     ColorTypeT |
+    JsonTypeT |
     ObjectTypeT |
     ValueTypeT |
     ArrayType | // eslint-disable-line no-use-before-define
@@ -37,6 +39,7 @@ export const NumberType = { kind: 'number' };
 export const StringType = { kind: 'string' };
 export const BooleanType = { kind: 'boolean' };
 export const ColorType = { kind: 'color' };
+export const JsonType = { kind: 'json' };
 export const ObjectType = { kind: 'object' };
 export const ValueType = { kind: 'value' };
 export const ErrorType = { kind: 'error' };
@@ -68,6 +71,7 @@ const valueMemberTypes = [
     StringType,
     BooleanType,
     ColorType,
+    JsonType,
     FormattedType,
     ObjectType,
     array(ValueType)

src/style-spec/reference/v8.json

@@ -2606,6 +2606,15 @@
           }
         }
       },
+      "to-json": {
+        "doc": "Converts the input value to a JSON. The input is converted to a JSON using the [`JSON.parse`](https://tc39.github.io/ecma262/#sec-json.parse) function of the ECMAScript Language Specification.",
+        "group": "Types",
+        "sdk-support": {
+          "basic functionality": {
+            "js": "0.52.0"
+          }
+        }
+      },
       "to-string": {
         "doc": "Converts the input value to a string. If the input is `null`, the result is `\"\"`. If the input is a boolean, the result is `\"true\"` or `\"false\"`. If the input is a number, it is converted to a string as specified by the [\"NumberToString\" algorithm](https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type) of the ECMAScript Language Specification. If the input is a color, it is converted to a string of the form `\"rgba(r,g,b,a)\"`, where `r`, `g`, and `b` are numerals ranging from 0 to 255, and `a` ranges from 0 to 1. Otherwise, the input is converted to a string in the format specified by the [`JSON.stringify`](https://tc39.github.io/ecma262/#sec-json.stringify) function of the ECMAScript Language Specification.",
         "group": "Types",

test/integration/expression-tests/to-json/2-ary/test.json

@@ -0,0 +1,10 @@
+{
+  "expression": ["to-json", ["get", "x"], ["get", "y"]],
+  "inputs": [[{}, {}]],
+  "expected": {
+    "compiled": {
+      "result": "error",
+      "errors": [{"key": "", "error": "Expected one argument."}]
+    }
+  }
+}

test/integration/expression-tests/to-json/basic/test.json

@@ -0,0 +1,21 @@
+{
+  "expression": ["to-json", ["get", "x"]],
+  "inputs": [
+    [{}, {"properties": {"x": "1"}}],
+    [{}, {"properties": {"x": "false"}}],
+    [{}, {"properties": {"x": "null"}}],
+    [{}, {"properties": {"x": "string"}}],
+    [{}, {"properties": {"x": "[1, 2]"}}],
+    [{}, {"properties": {"x": "{\"y\":1}"}}]
+  ],
+  "expected": {
+    "compiled": {
+      "result": "success",
+      "isFeatureConstant": false,
+      "isZoomConstant": true,
+      "type": "json"
+    },
+    "outputs": [1, false, null, {"error":{}}, [1, 2], {"y": 1}],
+    "serialized": ["to-json", ["get", "x"]]
+  }
+}