http请求options预请求的demo

# safe-buffer [![travis][travis-image]][travis-url] [![npm][npm-image]][npm-url] [![downloads][downloads-image]][downloads-url] [![javascript style guide][standard-image]][standard-url] [travis-image]: https://img.shields.io/travis/feross/safe-buffer/master.svg [travis-url]: https://travis-ci.org/feross/safe-buffer [npm-image]: https://img.shields.io/npm/v/safe-buffer.svg [npm-url]: https://npmjs.org/package/safe-buffer [downloads-image]: https://img.shields.io/npm/dm/safe-buffer.svg [downloads-url]: https://npmjs.org/package/safe-buffer [standard-image]: https://img.shields.io/badge/code_style-standard-brightgreen.svg [standard-url]: https://standardjs.com #### Safer Node.js Buffer API **Use the new Node.js Buffer APIs (`Buffer.from`, `Buffer.alloc`, `Buffer.allocUnsafe`, `Buffer.allocUnsafeSlow`) in all versions of Node.js.** **Uses the built-in implementation when available.** ## install ``` npm install safe-buffer ``` ## usage The goal of this package is to provide a safe replacement for the node.js `Buffer`. It's a drop-in replacement for `Buffer`. You can use it by adding one `require` line to the top of your node.js modules: ```js var Buffer = require('safe-buffer').Buffer // Existing buffer code will continue to work without issues: new Buffer('hey', 'utf8') new Buffer([1, 2, 3], 'utf8') new Buffer(obj) new Buffer(16) // create an uninitialized buffer (potentially unsafe) // But you can use these new explicit APIs to make clear what you want: Buffer.from('hey', 'utf8') // convert from many types to a Buffer Buffer.alloc(16) // create a zero-filled buffer (safe) Buffer.allocUnsafe(16) // create an uninitialized buffer (potentially unsafe) ``` ## api ### Class Method: Buffer.from(array) <!-- YAML added: v3.0.0 --> * `array` {Array} Allocates a new `Buffer` using an `array` of octets. ```js const buf = Buffer.from([0x62,0x75,0x66,0x66,0x65,0x72]); // creates a new Buffer containing ASCII bytes // ['b','u','f','f','e','r'] ``` A `TypeError` will be thrown if `array` is not an `Array`. ### Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]]) <!-- YAML added: v5.10.0 --> * `arrayBuffer` {ArrayBuffer} The `.buffer` property of a `TypedArray` or a `new ArrayBuffer()` * `byteOffset` {Number} Default: `0` * `length` {Number} Default: `arrayBuffer.length - byteOffset` When passed a reference to the `.buffer` property of a `TypedArray` instance, the newly created `Buffer` will share the same allocated memory as the TypedArray. ```js const arr = new Uint16Array(2); arr[0] = 5000; arr[1] = 4000; const buf = Buffer.from(arr.buffer); // shares the memory with arr; console.log(buf); // Prints: <Buffer 88 13 a0 0f> // changing the TypedArray changes the Buffer also arr[1] = 6000; console.log(buf); // Prints: <Buffer 88 13 70 17> ``` The optional `byteOffset` and `length` arguments specify a memory range within the `arrayBuffer` that will be shared by the `Buffer`. ```js const ab = new ArrayBuffer(10); const buf = Buffer.from(ab, 0, 2); console.log(buf.length); // Prints: 2 ``` A `TypeError` will be thrown if `arrayBuffer` is not an `ArrayBuffer`. ### Class Method: Buffer.from(buffer) <!-- YAML added: v3.0.0 --> * `buffer` {Buffer} Copies the passed `buffer` data onto a new `Buffer` instance. ```js const buf1 = Buffer.from('buffer'); const buf2 = Buffer.from(buf1); buf1[0] = 0x61; console.log(buf1.toString()); // 'auffer' console.log(buf2.toString()); // 'buffer' (copy is not changed) ``` A `TypeError` will be thrown if `buffer` is not a `Buffer`. ### Class Method: Buffer.from(str[, encoding]) <!-- YAML added: v5.10.0 --> * `str` {String} String to encode. * `encoding` {String} Encoding to use, Default: `'utf8'` Creates a new `Buffer` containing the given JavaScript string `str`. If provided, the `encoding` parameter identifies the character encoding. If not provided, `encoding` defaults to `'utf8'`. ```js const buf1 = Buffer.from('this is a tést'); console.log(buf1.toString()); // prints: this is a tést console.log(buf1.toString('ascii')); // prints: this is a tC)st const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex'); console.log(buf2.toString()); // prints: this is a tést ``` A `TypeError` will be thrown if `str` is not a string. ### Class Method: Buffer.alloc(size[, fill[, encoding]]) <!-- YAML added: v5.10.0 --> * `size` {Number} * `fill` {Value} Default: `undefined` * `encoding` {String} Default: `utf8` Allocates a new `Buffer` of `size` bytes. If `fill` is `undefined`, the `Buffer` will be *zero-filled*. ```js const buf = Buffer.alloc(5); console.log(buf); // <Buffer 00 00 00 00 00> ``` The `size` must be less than or equal to the value of `require('buffer').kMaxLength` (on 64-bit architectures, `kMaxLength` is `(2^31)-1`). Otherwise, a [`RangeError`][] is thrown. A zero-length Buffer will be created if a `size` less than or equal to 0 is specified. If `fill` is specified, the allocated `Buffer` will be initialized by calling `buf.fill(fill)`. See [`buf.fill()`][] for more information. ```js const buf = Buffer.alloc(5, 'a'); console.log(buf); // <Buffer 61 61 61 61 61> ``` If both `fill` and `encoding` are specified, the allocated `Buffer` will be initialized by calling `buf.fill(fill, encoding)`. For example: ```js const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64'); console.log(buf); // <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64> ``` Calling `Buffer.alloc(size)` can be significantly slower than the alternative `Buffer.allocUnsafe(size)` but ensures that the newly created `Buffer` instance contents will *never contain sensitive data*. A `TypeError` will be thrown if `size` is not a number. ### Class Method: Buffer.allocUnsafe(size) <!-- YAML added: v5.10.0 --> * `size` {Number} Allocates a new *non-zero-filled* `Buffer` of `size` bytes. The `size` must be less than or equal to the value of `require('buffer').kMaxLength` (on 64-bit architectures, `kMaxLength` is `(2^31)-1`). Otherwise, a [`RangeError`][] is thrown. A zero-length Buffer will be created if a `size` less than or equal to 0 is specified. The underlying memory for `Buffer` instances created in this way is *not initialized*. The contents of the newly created `Buffer` are unknown and *may contain sensitive data*. Use [`buf.fill(0)`][] to initialize such `Buffer` instances to zeroes. ```js const buf = Buffer.allocUnsafe(5); console.log(buf); // <Buffer 78 e0 82 02 01> // (octets will be different, every time) buf.fill(0); console.log(buf); // <Buffer 00 00 00 00 00> ``` A `TypeError` will be thrown if `size` is not a number. Note that the `Buffer` module pre-allocates an internal `Buffer` instance of size `Buffer.poolSize` that is used as a pool for the fast allocation of new `Buffer` instances created using `Buffer.allocUnsafe(size)` (and the deprecated `new Buffer(size)` constructor) only when `size` is less than or equal to `Buffer.poolSize >> 1` (floor of `Buffer.poolSize` divided by two). The default value of `Buffer.poolSize` is `8192` but can be modified. Use of this pre-allocated internal memory pool is a key difference between calling `Buffer.alloc(size, fill)` vs. `Buffer.allocUnsafe(size).fill(fill)`. Specifically, `Buffer.alloc(size, fill)` will *never* use the internal Buffer pool, while `Buffer.allocUnsafe(size).fill(fill)` *will* use the internal Buffer pool if `size` is less than or equal to half `Buffer.poolSize`. The difference is subtle but can be important when an application requires the additional performance that `Buffer.allocUnsafe(size)` provides. ### Class Method: Buffer.allocUnsafeSlow(size) <!-- YAML added: v5.10.0 --> * `size` {Number} Allocates a new *non-zero-filled* and non-pooled `Buffer` of `size` bytes. The `size` must be less than or equal to the value of `require('buffer').kMaxLength` (on 64-bit architectures, `kMaxLength` is `(2^31)-1`). Otherwise, a [`RangeError`][] is thrown. A zero-length Buffer will be created if a `size` l