Babel默认只转换新的JavaScript句法（syntax），而不转换新的API，比如Iterator、Generator、Set、Maps、Proxy、Reflect、Symbol、Promise等全局对象，以及一些定义在全局对象上的方法（比如Object.assign）都不会转码。Babel默认不转码的API非常多，详细清单可以查看definitions.js文件。
definitions.js文件内容如下：
1 export default { 2 builtins: { 3 Symbol: "symbol", 4 Promise: "promise", 5 Map: "map", 6 WeakMap: "weak-map", 7 Set: "set", 8 WeakSet: "weak-set", 9 Observable: "observable", 10 setImmediate: "set-immediate", 11 clearImmediate: "clear-immediate", 12 asap: "asap", 13 //parseFloat: "parse-float", // temporary disabled 14 //parseInt: "parse-int" // temporary disabled 15 }, 16 17 methods: { 18 Array: { 19 copyWithin: "array/copy-within", 20 entries: "array/entries", 21 every: "array/every", 22 fill: "array/fill", 23 filter: "array/filter", 24 findIndex: "array/find-index", 25 find: "array/find", 26 forEach: "array/for-each", 27 from: "array/from", 28 includes: "array/includes", 29 indexOf: "array/index-of", 30 //isArray: "array/is-array", // temporary disabled 31 join: "array/join", 32 keys: "array/keys", 33 lastIndexOf: "array/last-index-of", 34 map: "array/map", 35 of: "array/of", 36 reduceRight: "array/reduce-right", 37 reduce: "array/reduce", 38 some: "array/some", 39 sort: "array/sort", 40 splice: "array/splice", 41 values: "array/values", 42 }, 43 44 JSON: { 45 stringify: "json/stringify", 46 }, 47 48 Object: { 49 assign: "object/assign", 50 create: "object/create", 51 defineProperties: "object/define-properties", 52 defineProperty: "object/define-property", 53 entries: "object/entries", 54 freeze: "object/freeze", 55 getOwnPropertyDescriptor: "object/get-own-property-descriptor", 56 getOwnPropertyDescriptors: "object/get-own-property-descriptors", 57 getOwnPropertyNames: "object/get-own-property-names", 58 getOwnPropertySymbols: "object/get-own-property-symbols", 59 getPrototypeOf: "object/get-prototype-of", 60 isExtensible: "object/is-extensible", 61 isFrozen: "object/is-frozen", 62 isSealed: "object/is-sealed", 63 is: "object/is", 64 keys: "object/keys", 65 preventExtensions: "object/prevent-extensions", 66 seal: "object/seal", 67 setPrototypeOf: "object/set-prototype-of", 68 values: "object/values", 69 }, 70 71 Math: { 72 acosh: "math/acosh", 73 asinh: "math/asinh", 74 atanh: "math/atanh", 75 cbrt: "math/cbrt", 76 clz32: "math/clz32", 77 cosh: "math/cosh", 78 expm1: "math/expm1", 79 fround: "math/fround", 80 hypot: "math/hypot", 81 imul: "math/imul", 82 log10: "math/log10", 83 log1p: "math/log1p", 84 log2: "math/log2", 85 sign: "math/sign", 86 sinh: "math/sinh", 87 tanh: "math/tanh", 88 trunc: "math/trunc", 89 iaddh: "math/iaddh", 90 isubh: "math/isubh", 91 imulh: "math/imulh", 92 umulh: "math/umulh", 93 }, 94 95 Symbol: { 96 for: "symbol/for", 97 hasInstance: "symbol/has-instance", 98 isConcatSpreadable: "symbol/is-concat-spreadable", 99 iterator: "symbol/iterator", 100 keyFor: "symbol/key-for", 101 match: "symbol/match", 102 replace: "symbol/replace", 103 search: "symbol/search", 104 species: "symbol/species", 105 split: "symbol/split", 106 toPrimitive: "symbol/to-primitive", 107 toStringTag: "symbol/to-string-tag", 108 unscopables: "symbol/unscopables", 109 }, 110 111 String: { 112 at: "string/at", 113 codePointAt: "string/code-point-at", 114 endsWith: "string/ends-with", 115 fromCodePoint: "string/from-code-point", 116 includes: "string/includes", 117 matchAll: "string/match-all", 118 padStart: "string/pad-start", 119 padEnd: "string/pad-end", 120 raw: "string/raw", 121 repeat: "string/repeat", 122 startsWith: "string/starts-with", 123 trim: "string/trim", 124 trimLeft: "string/trim-left", 125 trimRight: "string/trim-right", 126 trimStart: "string/trim-start", 127 trimEnd: "string/trim-end", 128 }, 129 130 Number: { 131 EPSILON: "number/epsilon", 132 isFinite: "number/is-finite", 133 isInteger: "number/is-integer", 134 isNaN: "number/is-nan", 135 isSafeInteger: "number/is-safe-integer", 136 MAX_SAFE_INTEGER: "number/max-safe-integer", 137 MIN_SAFE_INTEGER: "number/min-safe-integer", 138 parseFloat: "number/parse-float", 139 parseInt: "number/parse-int", 140 }, 141 142 Reflect: { 143 apply: "reflect/apply", 144 construct: "reflect/construct", 145 defineProperty: "reflect/define-property", 146 deleteProperty: "reflect/delete-property", 147 getOwnPropertyDescriptor: "reflect/get-own-property-descriptor", 148 getPrototypeOf: "reflect/get-prototype-of", 149 get: "reflect/get", 150 has: "reflect/has", 151 isExtensible: "reflect/is-extensible", 152 ownKeys: "reflect/own-keys", 153 preventExtensions: "reflect/prevent-extensions", 154 setPrototypeOf: "reflect/set-prototype-of", 155 set: "reflect/set", 156 defineMetadata: "reflect/define-metadata", 157 deleteMetadata: "reflect/delete-metadata", 158 getMetadata: "reflect/get-metadata", 159 getMetadataKeys: "reflect/get-metadata-keys", 160 getOwnMetadata: "reflect/get-own-metadata", 161 getOwnMetadataKeys: "reflect/get-own-metadata-keys", 162 hasMetadata: "reflect/has-metadata", 163 hasOwnMetadata: "reflect/has-own-metadata", 164 metadata: "reflect/metadata", 165 }, 166 167 System: { 168 global: "system/global", 169 }, 170 171 Date: { 172 //now: "date/now" // temporary disabled 173 }, 174 175 Function: { 176 // Warning: /virtual/ method - prototype, not static, version 177 //bind: "function/virtual/bind" // temporary disabled 178 }, 179 }, 180 };
不支持的API需要引入pollify～
参考文章https://shenbao.github.io/ishehui/html/React/Babel%E4%BD%BF%E7%94%A8%E6%8C%87%E5%8D%97.html