CachedIterable copied from fluent 0.6.4

.gitignore

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+node_modules
+cached-iterable.js
+compat.js

.npmignore

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+test
+Makefile
+*_config.js
+eslint_*

.travis.yml

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+sudo: false
+language: node_js
+script: make all
+node_js: "8.9"
+cache:
+  directories: node_modules

Makefile

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+export SHELL := /bin/bash
+export PATH  := $(CURDIR)/node_modules/.bin:$(PATH)
+
+SOURCES := $(wildcard src/*)
+VERSION := $(shell node -pe "require('./package.json').version")
+OK := \033[32;01m✓\033[0m
+
+PACKAGE := cached-iterable
+GLOBAL  := CachedIterable
+
+# The default target.
+all: lint test build
+
+lint:
+	@eslint --config $(CURDIR)/eslint_src.json --max-warnings 0 src/
+	@eslint --config $(CURDIR)/eslint_test.json --max-warnings 0 test/
+	@echo -e " $(OK) $@"
+
+.PHONY: test
+test:
+	@mocha --recursive --ui tdd \
+	    --require mocha_config \
+	    test/**/*_test.js
+
+build: $(PACKAGE).js compat.js
+
+$(PACKAGE).js: $(SOURCES)
+	@rollup $(CURDIR)/src/index.mjs \
+	    --config $(CURDIR)/bundle_config.js \
+	    --banner "/* $(PACKAGE)@$(VERSION) */" \
+	    --amd.id $(PACKAGE) \
+	    --name $(GLOBAL) \
+	    --output.file $@
+	@echo -e " $(OK) $@ built"
+
+compat.js: $(SOURCES)
+	@rollup $(CURDIR)/src/index.mjs \
+	    --config $(CURDIR)/compat_config.js \
+	    --banner "/* $(PACKAGE)@$(VERSION) */" \
+	    --amd.id $(PACKAGE) \
+	    --name $(GLOBAL) \
+	    --output.file $@
+	@echo -e " $(OK) $@ built"
+
+clean:
+	@rm -f $(PACKAGE).js compat.js
+	@echo -e " $(OK) clean"

README.md

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+# cached-iterable
+
+`cached-iterable` exposes the `CachedItearble` class which implements the
+[iterable protocol][].
+
+You can wrap any iterable in these classes to create a new iterable which
+caches the yielded elements. This is useful for iterating over an iterable many
+times without depleting it.
+
+[iterable protocol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#The_iterable_protocol
+
+## Installation
+
+`cached-iterable` can be used both on the client-side and the server-side.  You
+can install it from the npm registry or use it as a standalone script (as the
+`CachedIterable` global).
+
+    npm install cached-iterable
+
+## How to use
+
+```js
+import assert from "assert";
+import {CachedIterable} from "cached-iterable";
+
+function * countdown(i) {
+    while (i--) {
+        yield i;
+    }
+}
+
+let numbers = new CachedIterable(countdown(3));
+
+// `numbers` can be iterated over multiple times.
+assert.deepEqual([...numbers], [3, 2, 1, 0]);
+assert.deepEqual([...numbers], [3, 2, 1, 0]);
+```
+
+## Compatibility
+
+For legacy browsers, the `compat` build has been transpiled using Babel's [env
+preset][]. It requires the regenerator runtime provided by [babel-polyfill][].
+
+```javascript
+import {CachedIterable} from 'cached-iterable/compat';
+```
+
+[env preset]: https://babeljs.io/docs/plugins/preset-env/
+[babel-polyfill]: https://babeljs.io/docs/usage/polyfill/

babel_config.js

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+export default {
+  babelrc: false,
+  presets: [
+    ["@babel/preset-env", {
+      // Cf. https://github.com/rollup/rollup-plugin-babel#modules
+      modules: false,
+      targets: {
+        browsers: [
+          ">1%",
+          "last 4 versions",
+          "Firefox ESR",
+          "not ie < 9"
+        ]
+      }
+    }]
+  ]
+};

bundle_config.js

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+export default {
+  output: {
+    format: 'umd',
+  },
+  acorn: {
+    ecmaVersion: 9,
+  },
+};

compat_config.js

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+import babel from 'rollup-plugin-babel';
+import babelConfig from './babel_config';
+
+export default {
+  output: {
+    format: 'umd'
+  },
+  plugins: [
+    babel(babelConfig),
+  ],
+};

eslint_src.json

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+{
+    "parser": "babel-eslint",
+    "parserOptions": {
+        "ecmaVersion": 9,
+        "sourceType": "module"
+    },
+    "env": {
+        "es6": true
+    },
+    "rules": {
+        "valid-typeof": 2,
+        "no-console": 1,
+        "no-dupe-args": 2,
+        "no-dupe-keys": 2,
+        "no-duplicate-case": 2,
+        "no-extra-semi": 1,
+        "no-func-assign": 2,
+        "func-call-spacing": [
+            2,
+            "never"
+        ],
+        "no-unreachable": 1,
+        "no-unexpected-multiline": 2,
+        "no-sparse-arrays": 2,
+        "complexity": [
+            1,
+            18
+        ],
+        "dot-location": [
+            2,
+            "property"
+        ],
+        "no-implied-eval": 2,
+        "no-loop-func": 2,
+        "no-magic-numbers": [
+            1,
+            {
+                "ignore": [
+                    -1,
+                    0,
+                    1,
+                    2
+                ]
+            }
+        ],
+        "no-useless-call": 2,
+        "no-useless-concat": 2,
+        "no-delete-var": 2,
+        "no-shadow": 2,
+        "no-undef": 2,
+        "no-undef-init": 2,
+        "no-unused-vars": 1,
+        "consistent-return": 1,
+        "curly": [
+            2,
+            "multi-line"
+        ],
+        "eqeqeq": 2,
+        "no-extend-native": 2,
+        "no-global-assign": 2,
+        "no-extra-bind": 2,
+        "no-redeclare": 2,
+        "array-bracket-spacing": 2,
+        "brace-style": [
+            1,
+            "1tbs"
+        ],
+        "no-mixed-spaces-and-tabs": 2,
+        "no-tabs": 2,
+        "prefer-arrow-callback": 1,
+        "prefer-const": 2,
+        "prefer-template": 2,
+        "prefer-spread": 2,
+        "quotes": [
+            2,
+            "double",
+            {
+                "avoidEscape": true
+            }
+        ],
+        "max-depth": [
+            2,
+            6
+        ],
+        "max-len": [
+            2,
+            80
+        ],
+        "no-nested-ternary": 2,
+        "no-unneeded-ternary": 2,
+        "strict": [
+            2,
+            "global"
+        ],
+        "indent": [
+            2,
+            4,
+            {
+                "SwitchCase": 1
+            }
+        ],
+        "no-useless-escape": 2,
+        "no-duplicate-imports": 2,
+        "no-unsafe-negation": 2,
+        "no-use-before-define": [
+            2,
+            {
+                "functions": false,
+                "classes": false
+            }
+        ],
+        "space-infix-ops": 2,
+        "no-constant-condition": [
+            1,
+            {
+                "checkLoops": false
+            }
+        ],
+        "no-empty": 1,
+        "use-isnan": 2,
+        "dot-notation": 2,
+        "no-caller": 2,
+        "no-else-return": 2,
+        "no-eval": 2,
+        "no-implicit-globals": 2,
+        "no-iterator": 2,
+        "no-multi-spaces": 2,
+        "no-multi-str": 2,
+        "no-octal": 2,
+        "no-proto": 2,
+        "no-sequences": 2,
+        "block-spacing": 2,
+        "eol-last": 2,
+        "key-spacing": 2,
+        "no-trailing-spaces": 2,
+        "prefer-rest-params": 2,
+        "no-cond-assign": 2,
+        "keyword-spacing": 2,
+        "arrow-parens": [
+            2,
+            "as-needed"
+        ],
+        "no-useless-return": 2,
+        "semi": 2,
+        "spaced-comment": [
+            2,
+            "always"
+        ]
+    }
+}

eslint_test.json

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+{
+    "parser": "babel-eslint",
+    "parserOptions": {
+        "ecmaVersion": 9,
+        "sourceType": "module",
+        "ecmaFeatures": {
+            "jsx": true
+        }
+    },
+    "env": {
+        "es6": true,
+        "node": true,
+        "mocha": true
+    },
+    "plugins": [
+        "mocha"
+    ],
+    "rules": {
+        "mocha/no-exclusive-tests": "error",
+        "mocha/no-identical-title": "error"
+    }
+}

mocha_config.js

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+"use strict";
+
+require("@babel/polyfill");
+require("@babel/register")({
+  plugins: [
+    "@babel/plugin-proposal-async-generator-functions",
+    "@babel/plugin-transform-modules-commonjs"
+  ]
+});