_

语法：

_(value)

描述：

创建一个lodash对象，包装value后的对象启用隐式方法链。返回的数组、集合、方法相互之间能够链式调用。检索唯一值或返回原始值会自动解除链条并返回计算后的值，否则需要调用_#value方法解除链(即获得计算结果)。 显式链式调用，在任何情况下需要先用_#value解除链后，才能使用_.chain开启。

链式方法是惰性计算的，直到隐式或者显式调用了_#value才会执行计算。

惰性计算接受几种支持 shortcut fusion 的方法， shortcut fusion 是一种通过合并链式 iteratee 调用从而大大降低迭代的次数以提高执行性能的方式。 部分链有资格 shortcut fusion，如果它至少有超过200个元素的数组和任何只接受一个参数的 iteratees。 触发的方式是任何一个 shortcut fusion 有了变化。

链式方法支持定制版本，只要 _#value 包含或者间接包含在版本中。

除了 lodash 的自身方法，包装后的对象还支持 Array 和 String 的方法。

支持 Array 的方法: concat, join, pop, push, shift, sort, splice, 和 unshift

支持 String 的方法: replace 和 split

支持 shortcut fusion 的方法: at, compact, drop, dropRight, dropWhile, filter, find, findLast, head, initial, last, map, reject, reverse, slice, tail, take, takeRight, takeRightWhile, takeWhile, 和 toArray

支持 链式调用 的方法: after, ary, assign, assignIn, assignInWith, assignWith, at, before, bind, bindAll, bindKey, castArray, chain, chunk, commit, compact, concat, conforms, constant, countBy, create, curry, debounce, defaults, defaultsDeep, defer, delay, difference, differenceBy, differenceWith, drop, dropRight, dropRightWhile, dropWhile, extend, extendWith, fill, filter, flatMap, flatMapDeep, flatMapDepth, flatten, flattenDeep, flattenDepth, flip, flow, flowRight, fromPairs, functions, functionsIn, groupBy, initial, intersection, intersectionBy, intersectionWith, invert, invertBy, invokeMap, iteratee, keyBy, keys, keysIn, map, mapKeys, mapValues, matches, matchesProperty, memoize, merge, mergeWith, method, methodOf, mixin, negate, nthArg, omit, omitBy, once, orderBy, over, overArgs, overEvery, overSome, partial, partialRight, partition, pick, pickBy, plant, property, propertyOf, pull, pullAll, pullAllBy, pullAllWith, pullAt, push, range, rangeRight, rearg, reject, remove, rest, reverse, sampleSize, set, setWith, shuffle, slice, sort, sortBy, splice, spread, tail, take, takeRight, takeRightWhile, takeWhile, tap, throttle, thru, toArray, toPairs, toPairsIn, toPath, toPlainObject, transform, unary, union, unionBy, unionWith, uniq, uniqBy, uniqWith, unset, unshift, unzip, unzipWith, update, updateWith, values, valuesIn, without, wrap, xor, xorBy, xorWith, zip, zipObject, zipObjectDeep, and zipWith

默认 不 支持 链式调用 的方法: add, attempt, camelCase, capitalize, ceil, clamp, clone, cloneDeep, cloneDeepWith, cloneWith, conformsTo, deburr, defaultTo, divide, each, eachRight, endsWith, eq, escape, escapeRegExp, every, find, findIndex, findKey, findLast, findLastIndex, findLastKey, first, floor, forEach, forEachRight, forIn, forInRight, forOwn, forOwnRight, get, gt, gte, has, hasIn, head, identity, includes, indexOf, inRange, invoke, isArguments, isArray, isArrayBuffer, isArrayLike, isArrayLikeObject, isBoolean, isBuffer, isDate, isElement, isEmpty, isEqual, isEqualWith, isError, isFinite, isFunction, isInteger, isLength, isMap, isMatch, isMatchWith, isNaN, isNative, isNil, isNull, isNumber, isObject, isObjectLike, isPlainObject, isRegExp, isSafeInteger, isSet, isString, isUndefined, isTypedArray, isWeakMap, isWeakSet, join, kebabCase, last, lastIndexOf, lowerCase, lowerFirst, lt, lte, max, maxBy, mean, meanBy, min, minBy, multiply, noConflict, noop, now, nth, pad, padEnd, padStart, parseInt, pop, random, reduce, reduceRight, repeat, result, round, runInContext, sample, shift, size, snakeCase, some, sortedIndex, sortedIndexBy, sortedLastIndex, sortedLastIndexBy, startCase, startsWith, stubArray, stubFalse, stubObject, stubString, stubTrue, subtract, sum, sumBy, template, times, toFinite, toInteger, toJSON, toLength, toLower, toNumber, toSafeInteger, toString, toUpper, trim, trimEnd, trimStart, truncate, unescape, uniqueId, upperCase, upperFirst, value, and words

参数：

• value (*): 需要被包装为 lodash 实例的值。

返回值：

• (Object): 返回 lodash 包装后的实例。

例子：

function square(n) {
  return n * n;
}

var wrapped = _([1, 2, 3]);

// Returns an unwrapped value.
wrapped.reduce(_.add);
// => 6

// Returns a wrapped value.
var squares = wrapped.map(square);

_.isArray(squares);
// => false

_.isArray(squares.value());
// => true