The challenge hint says: it use eval and want from us to print " bcactf  " to a console:
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This is the Payload : 
`$={}+!1;$[2]+$[5]+$[16]+$[5]+$[6]+$[15];`
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"**$**" Variable Contain "**{}**" and "**!1**" concatenated, and will translated on Javascript to:
`{} `  // ----> "[object Object]"
`!1`  // ----> "false"
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Combine them to use it as a variable with indexing []:
`$={}+!1;`      // we define "**$**" variable with content : **[object Object]false**+`$[2] `  //b  is secend index on [object Object]false+`$[5] `  //c+`$[16] `  //a+`$[5] `  //c+`$[6]`   //t+`$[15] `  //f
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PoC:
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└─ nc misc.bcactf.com 49156Get the flag by making the calculator output "bcactf"!
`> $={}+!1;$[2]+$[5]+$[16]+$[5]+$[6]+$[15];`
Result: bcactfCongrats! The flag is bcactf{G00D_EV4LUAT1ON}
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> Saudi Team