support step arguments

src/buildin.js

@@ -117,12 +117,37 @@ module.exports = Object.freeze({
 
         return value !== undefined ? fn(value) : value;
     },
-    slice: function(value, from, to) {
-        if (Array.isArray(value) || typeof value === 'string') {
-            return value.slice(from, to);
+    slice: function(value, from, to, step) {
+        if (!Array.isArray(value) && typeof value !== 'string') {
+            return value;
         }
 
-        return value;
+        if (step && step !== 1) {
+            const result = [];
+            const reverse = step < 0;
+
+            step *= Math.sign(step);
+
+            if (from < 0) {
+                from = value.length + from;
+            }
+
+            if (to < 0) {
+                to = value.length + to;
+            }
+
+            for (let i = from; i < to; i += step) {
+                result.push(value[i]);
+            }
+
+            if (reverse) {
+                result.reverse();
+            }
+
+            return result;
+        }
+
+        return value.slice(from, to);
     },
     recursive: function(value, getter) {
         const result = new Set();

src/parser.js

@@ -256,10 +256,15 @@ const grammar = {
             ['.. SYMBOL', code`fn.recursive(/*@2*/current, "$2")`],
             ['..( block )', code`fn.recursive(current, current => { $2 })`],
             ['.[ block ]', code`fn.filter(current, current => { $2 })`],
-            ['[ e : e ]', code`fn.slice(current, $2, $4)`],
-            ['[ : e ]', code`fn.slice(current, 0, $3)`],
-            ['[ e : ]', code`fn.slice(current, $2, current.legnth)`],
-            ['[ : ]', code`fn.slice(current, 0, current.legnth)`]
+            ['[ e : e ]', code`fn.slice(current, $2, $4, 1)`],
+            ['[ e : e : e ]', code`fn.slice(current, $2, $4, $6)`],
+            ['[ : e ]', code`fn.slice(current, 0, $3, 1)`],
+            ['[ : e : e ]', code`fn.slice(current, 0, $3, $5)`],
+            ['[ e : ]', code`fn.slice(current, $2, current.length, 1)`],
+            ['[ e : : e ]', code`fn.slice(current, $2, current.length, $5)`],
+            ['[ : ]', code`fn.slice(current, 0, current.length, 1)`],
+            ['[ : : e ]', code`fn.slice(current, 0, current.length, $4)`],
+            ['[ : : ]', code`fn.slice(current, 0, current.length, 1)`]
         ],
 
         relativePath: [
@@ -271,10 +276,15 @@ const grammar = {
             ['query ..( block )', code`fn.recursive($1, current => { $3 })`],
             ['query .[ block ]', code`fn.filter($1, current => { $3 })`],
             ['query [ e ]', code`fn.map($1, $3)`],
-            ['query [ e : e ]', code`fn.slice($1, $3, $5)`],
-            ['query [ : e ]', code`fn.slice($1, 0, $4)`],
-            ['query [ e : ]', code`fn.slice($1, $3, $1.length)`],
-            ['query [ : ]', code`fn.slice($1, 0, $1.length)`]
+            ['query [ e : e ]', code`fn.slice($1, $3, $5, 1)`],
+            ['query [ e : e : e ]', code`fn.slice($1, $3, $5, $7)`],
+            ['query [ : e ]', code`fn.slice($1, 0, $4, 1)`],
+            ['query [ : e : e ]', code`fn.slice($1, 0, $4, $6)`],
+            ['query [ e : ]', code`fn.slice($1, $3, $1.length, 1)`],
+            ['query [ e : : e ]', code`fn.slice($1, $2, $1.length, $6)`],
+            ['query [ : ]', code`fn.slice($1, 0, $1.length, 1)`],
+            ['query [ : : e ]', code`fn.slice($1, 0, $1.length, $5)`],
+            ['query [ : : ]', code`fn.slice($1, 0, $1.length, 1)`]
         ],
 
         arguments: [

test/slice.js

@@ -3,9 +3,8 @@ const data = require('./fixture/simple');
 const query = require('../src');
 
 // https://github.com/tc39/proposal-slice-notation/blob/master/README.md
-// without a `step` argument
 
-describe('size | [N:N]', () => {
+describe('slice | [from:to]', () => {
     it('should be a data root', () => {
         assert.deepEqual(
             query('[0:3]')(data),
@@ -63,3 +62,127 @@ describe('size | [N:N]', () => {
         );
     });
 });
+
+describe('[from:to:step]', () => {
+    it('should be a data root', () => {
+        const fixture = data.slice(0, 3);
+
+        assert.deepEqual(
+            query('[0:3:2]')(data),
+            [fixture[0], fixture[2]]
+        );
+    });
+
+    it('should be a subquery', () => {
+        const fixture = data.slice(0, 3).map(i => i.filename);
+
+        assert.deepEqual(
+            query('filename[0:3:2]')(data),
+            [fixture[0], fixture[2]]
+        );
+    });
+
+    it('nagative from/to from root', () => {
+        const fixture = data.slice(-4, -1);
+
+        assert.deepEqual(
+            query('[-4:-1:2]')(data),
+            [fixture[0], fixture[2]]
+        );
+    });
+
+    it('nagative from/to from subquery', () => {
+        const fixture = data.slice(-4, -1).map(i => i.filename);
+
+        assert.deepEqual(
+            query('filename[-4:-1:2]')(data),
+            [fixture[0], fixture[2]]
+        );
+    });
+
+    it('reverse from root', () => {
+        const fixture = data.slice(0, 3);
+
+        assert.deepEqual(
+            query('[0:3:-1]')(data),
+            [fixture[2], fixture[1], fixture[0]]
+        );
+    });
+
+    it('reverse from subquery', () => {
+        const fixture = data.slice(0, 3).map(i => i.filename);
+
+        assert.deepEqual(
+            query('filename[0:3:-1]')(data),
+            [fixture[2], fixture[1], fixture[0]]
+        );
+    });
+
+    it('reverse with nagative from/to from root', () => {
+        const fixture = data.slice(-4, -1);
+
+        assert.deepEqual(
+            query('[-4:-1:-2]')(data),
+            [fixture[2], fixture[0]]
+        );
+    });
+
+    it('reverse with nagative from/to from subquery', () => {
+        const fixture = data.slice(-4, -1).map(i => i.filename);
+
+        assert.deepEqual(
+            query('filename[-4:-1:-2]')(data),
+            [fixture[2], fixture[0]]
+        );
+    });
+
+    it('negative from root', () => {
+        const fixture = data.slice(0, 3);
+
+        assert.deepEqual(
+            query('[0:3:-2]')(data),
+            [fixture[2], fixture[0]]
+        );
+    });
+
+    it('negative from subquery', () => {
+        const fixture = data.slice(0, 3).map(i => i.filename);
+
+        assert.deepEqual(
+            query('filename[0:3:-2]')(data),
+            [fixture[2], fixture[0]]
+        );
+    });
+
+    it('pass from/to args from root', () => {
+        const fixture = data.slice();
+
+        assert.deepEqual(
+            query('[::-2]')(data),
+            [fixture[6], fixture[4], fixture[2], fixture[0]]
+        );
+    });
+
+    it('pass from/to args from query', () => {
+        const fixture = data.map(i => i.filename);
+
+        assert.deepEqual(
+            query('filename[::-2]')(data),
+            [fixture[6], fixture[4], fixture[2], fixture[0]]
+        );
+    });
+
+    it('pass all args from root', () => {
+        assert.deepEqual(
+            query('[::]')(data),
+            data.slice()
+        );
+    });
+
+    it('pass all args from query', () => {
+        assert.deepEqual(
+            query('filename[::]')(data),
+            data.map(i => i.filename)
+        );
+    });
+});