wxml中不能用typeof判断undefined？

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wxml中不能用typeof判断undefined？Can't use typeof to judge undefined in wxml?

data:{  // nothing }

<view wx:if="{{typeof(foo)!=='undefined'}}">hello   view>

foo!==undefined // truetypeof(foo)!=='undefined' // falsetypeof(foo)!==undefined // false

typeof(foo)!=='undefined' // true

Data: {/ / nothing} wxml < view Wx: if = {{typeof (foo)! = = = = ='undefined '}} > > Hello > in wxml, the output of typeof (foo) in wxml is undefined, but the expression does not hold. In wxml, foo! = = undefined / / truetypeof (foo)! = = = = = = = undefined '/ / false type (foo)! = = undefined / / false JS is true. Why is it inconsistent in wxml? typeof(foo)!=='undefined' // true

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