README example for feature filter is no more working/valid with style-spec 10.0.x

[lamuertepeluda]

The code from the readme of style-spec should now change from

var ff = require('feature-filter');

// will match a feature with class of street_limited,
// AND an admin_level less than or equal to 3,
// that's NOT a polygon.
var filter = [
    "all",
    ["==", "class", "street_limited"],
    ["<=", "admin_level", 3],
    ["!=", "$type", "Polygon"]
]

// will match a feature that has a class of
// wetland OR wetland_noveg.
// ["in", "class", "wetland", "wetland_noveg"]

// testFilter will be a function that returns a boolean
var testFilter = ff(filter);

// Layer feature that you're testing. Must have type
// and properties keys.
var feature = {
    type: 2,
    properties: {
       class: "street_limited"
       admin_level: 1
    }
};

// will return a boolean based on whether the feature matched the filter
return testFilter(feature);

to

var ff = require('feature-filter');

// will match a feature with class of street_limited,
// AND an admin_level less than or equal to 3,
// that's NOT a polygon.
var filter = [
    "all",
    ["==", "class", "street_limited"],
    ["<=", "admin_level", 3],
    ["!=", "$type", "Polygon"]
]

// will match a feature that has a class of
// wetland OR wetland_noveg.
// ["in", "class", "wetland", "wetland_noveg"]

// testFilter will be a function that returns a boolean
var testFilter = ff(filter);

// Layer feature that you're testing. Must have type
// and properties keys.
var feature = {
    type: 2,
    properties: {
-       class: "street_limited"
+       class: "street_limited", //WARNING: MISSING COMMA
       admin_level: 1
    }
};

// will return a boolean based on whether the feature matched the filter
-return testFilter(feature);
+return testFilter(null, feature); // <-- MAIN CHANGE HERE

The reason why last line is now wrong and would always return false is that a second, unused argument ('g') has been added to the generated filter at this line .

I'm sure there has to be a good reason for this 😄 but I'd like some explanation.

[lamuertepeluda]

See also #5502 for usage within a browser

[jfirebaugh]

Thanks for reporting. We added this parameter in order to support filter expressions that use the zoom expression operator.