const mock = ( ) => { let storage = { } ; return { getItem : key => ( key in storage ? storage [ key ] : null ) , setItem : ( key , value ) => ( storage [ key ] = value || "" ) , removeItem : key => delete storage [ key ] , clear : ( ) => ( storage = { } ) } ; } ; Object . defineProperty ( window , "localStorage" , { value : mock ( ) } ) ; Object . defineProperty ( window , "sessionStorage" , { value : mock ( ) } ) ; Object . defineProperty ( window , "getComputedStyle" , { value : ( ) => [ "-webkit-appearance" ] } ) ;